GROFF_OUT(5)              OpenBSD Programmer's Manual             GROFF_OUT(5)



NAME
       groff_out - groff intermediate output format

DESCRIPTION
       This manual page describes the intermediate output format of the GNU
       roff(7) text processing system groff(1).  This output is produced by a
       run of the GNU troff(1) program.  It contains already all device-
       specific information, but it is not yet fed into a device postprocessor
       program.

       As the GNU roff processor groff(1) is a wrapper program around troff
       that automatically calls a postprocessor, this output does not show up
       normally.  This is why it is called intermediate within the groff
       system.  The groff program provides the option -Z to inhibit
       postprocessing, such that the produced intermediate output is sent to
       standard output just like calling troff manually.

       In this document, the term troff output describes what is output by the
       GNU troff program, while intermediate output refers to the language
       that is accepted by the parser that prepares this output for the
       postprocessors.  This parser is smarter on whitespace and implements
       obsolete elements for compatibility, otherwise both formats are the
       same.  Both formats can be viewed directly with gxditview(1).

       The main purpose of the intermediate output concept is to facilitate
       the development of postprocessors by providing a common programming
       interface for all devices.  It has a language of its own that is
       completely different from the groff(7) language.  While the groff
       language is a high-level programming language for text processing, the
       intermediate output language is a kind of low-level assembler language
       by specifying all positions on the page for writing and drawing.

       The pre-groff roff versions are denoted as classical troff.  The
       intermediate output produced by groff is fairly readable, while
       classical troff output was hard to understand because of strange habits
       that are still supported, but not used any longer by GNU troff.

LANGUAGE CONCEPTS
       During the run of troff, the roff input is cracked down to the
       information on what has to be printed at what position on the intended
       device.  So the language of the intermediate output format can be quite
       small.  Its only elements are commands with or without arguments.  In
       this document, the term "command" always refers to the intermediate
       output language, never to the roff language used for document
       formatting.  There are commands for positioning and text writing, for
       drawing, and for device controlling.

   Separation
       Classical troff output had strange requirements on whitespace.  The
       groff output parser, however, is smart about whitespace by making it
       maximally optional.  The whitespace characters, i.e., the tab, space,
       and newline characters, always have a syntactical meaning.  They are
       never printable because spacing within the output is always done by
       positioning commands.

       Any sequence of space or tab characters is treated as a single
       syntactical space.  It separates commands and arguments, but is only
       required when there would occur a clashing between the command code and
       the arguments without the space.  Most often, this happens when
       variable length command names, arguments, argument lists, or command
       clusters meet.  Commands and arguments with a known, fixed length need
       not be separated by syntactical space.

       A line break is a syntactical element, too.  Every command argument can
       be followed by whitespace, a comment, or a newline character.  Thus a
       syntactical line break is defined to consist of optional syntactical
       space that is optionally followed by a comment, and a newline
       character.

       The normal commands, those for positioning and text, consist of a
       single letter taking a fixed number of arguments.  For historical
       reasons, the parser allows to stack such commands on the same line, but
       fortunately, in groff intermediate output, every command with at least
       one argument is followed by a line break, thus providing excellent
       readability.

       The other commands -- those for drawing and device controlling -- have
       a more complicated structure; some recognize long command names, and
       some take a variable number of arguments.  So all D and x commands were
       designed to request a syntactical line break after their last argument.
       Only one command, `x X' has an argument that can stretch over several
       lines, all other commands must have all of their arguments on the same
       line as the command, i.e., the arguments may not be split by a line
       break.

       Empty lines, i.e., lines containing only space and/or a comment, can
       occur everywhere.  They are just ignored.

   Argument Units
       Some commands take integer arguments that are assumed to represent
       values in a measurement unit, but the letter for the corresponding
       scale indicator is not written with the output command arguments; see
       groff(7) and the groff info file for more on this topic.  Most commands
       assume the scale indicator u, the basic unit of the device, some use z,
       the scaled point unit of the device, while others, such as the color
       commands expect plain integers.  Note that these scale indicators are
       relative to the chosen device.  They are defined by the parameters
       specified in the device's DESC file; see groff_font(5).

       Note that single characters can have the eighth bit set, as can the
       names of fonts and special characters (this is, glyphs).  The names of
       glyphs and fonts can be of arbitrary length.  A glyph that is to be
       printed will always be in the current font.

       A string argument is always terminated by the next whitespace character
       (space, tab, or newline); an embedded # character is regarded as part
       of the argument, not as the beginning of a comment command.  An integer
       argument is already terminated by the next non-digit character, which
       then is regarded as the first character of the next argument or
       command.

   Document Parts
       A correct intermediate output document consists of two parts, the
       prologue and the body.

       The task of the prologue is to set the general device parameters using
       three exactly specified commands.  The groff prologue is guaranteed to
       consist of the following three lines (in that order):

              x T device
              x res n h v
              x init

       with the arguments set as outlined in the section Device Control
       Commands.  However, the parser for the intermediate output format is
       able to swallow additional whitespace and comments as well.

       The body is the main section for processing the document data.
       Syntactically, it is a sequence of any commands different from the ones
       used in the prologue.  Processing is terminated as soon as the first
       x stop command is encountered; the last line of any groff intermediate
       output always contains such a command.

       Semantically, the body is page oriented.  A new page is started by a
       p command.  Positioning, writing, and drawing commands are always done
       within the current page, so they cannot occur before the first
       p command.  Absolute positioning (by the H and V commands) is done
       relative to the current page, all other positioning is done relative to
       the current location within this page.

COMMAND REFERENCE
       This section describes all intermediate output commands, the classical
       commands as well as the groff extensions.

   Comment Command
       #anything<end-of-line>
              A comment.  Ignore any characters from the # character up to the
              next newline character.

       This command is the only possibility for commenting in the intermediate
       output.  Each comment can be preceded by arbitrary syntactical space;
       every command can be terminated by a comment.

   Simple Commands
       The commands in this subsection have a command code consisting of a
       single character, taking a fixed number of arguments.  Most of them are
       commands for positioning and text writing.  These commands are smart
       about whitespace.  Optionally, syntactical space can be inserted
       before, after, and between the command letter and its arguments.  All
       of these commands are stackable, i.e., they can be preceded by other
       simple commands or followed by arbitrary other commands on the same
       line.  A separating syntactical space is only necessary when two
       integer arguments would clash or if the preceding argument ends with a
       string argument.

       C xxx<white-space>
              Print a glyph (special character) named xxx.  The trailing
              syntactical space or line break is necessary to allow glyph
              names of arbitrary length.  The glyph is printed at the current
              print position; the glyph's size is read from the font file.
              The print position is not changed.

       c c    Print glyph with single-letter name c at the current print
              position; the glyph's size is read from the font file.  The
              print position is not changed.

       f n    Set font to font number n (a non-negative integer).

       H n    Move right to the absolute vertical position n (a non-negative
              integer in basic units u) relative to left edge of current page.

       h n    Move n (a non-negative integer) basic units u horizontally to
              the right.  [CSTR #54] allows negative values for n also, but
              groff doesn't use this.

       m color-scheme [component ...]
              Set the color for text (glyphs), line drawing, and the outline
              of graphic objects using different color schemes; the analoguous
              command for the filling color of graphic objects is DF.  The
              color components are specified as integer arguments between 0
              and 65536.  The number of color components and their meaning
              vary for the different color schemes.  These commands are
              generated by the groff escape sequence \m.  No position
              changing.  These commands are a groff extension.

              mc cyan magenta yellow
                     Set color using the CMY color scheme, having the 3 color
                     components cyan, magenta, and yellow.

              md     Set color to the default color value (black in most
                     cases).  No component arguments.

              mg gray
                     Set color to the shade of gray given by the argument, an
                     integer between 0 (black) and 65536 (white).

              mk cyan magenta yellow black
                     Set color using the CMYK color scheme, having the 4 color
                     components cyan, magenta, yellow, and black.

              mr red green blue
                     Set color using the RGB color scheme, having the 3 color
                     components red, green, and blue.

       N n    Print glyph with index n (an integer, normally non-negative) of
              the current font.  The print position is not changed.  If
              -T html or -T xhtml is used, negative values are emitted also to
              indicate an unbreakable space with given width.  For example,
              N -193 represents an unbreakable space which has a width of
              193u.  This command is a groff extension.

       n b a  Inform the device about a line break, but no positioning is done
              by this command.  In classical troff, the integer arguments b
              and a informed about the space before and after the current line
              to make the intermediate output more human readable without
              performing any action.  In groff, they are just ignored, but
              they must be provided for compatibility reasons.

       p n    Begin a new page in the outprint.  The page number is set to n.
              This page is completely independent of pages formerly processed
              even if those have the same page number.  The vertical position
              on the outprint is automatically set to 0.  All positioning,
              writing, and drawing is always done relative to a page, so a
              p command must be issued before any of these commands.

       s n    Set point size to n scaled points (this is unit z in GNU troff).
              Classical troff used the unit points (p) instead; see section
              COMPATIBILITY.

       t xyz...<white-space>
       t xyz... dummy-arg<white-space>
              Print a word, i.e., a sequence of glyphs with single-letter
              names x, y, z, etc., terminated by a space character or a line
              break; an optional second integer argument is ignored (this
              allows the formatter to generate an even number of arguments).
              The first glyph should be printed at the current position, the
              current horizontal position should then be increased by the
              width of the first glyph, and so on for each glyph.  The widths
              of the glyph are read from the font file, scaled for the current
              point size, and rounded to a multiple of the horizontal
              resolution.  Special characters (glyphs with names longer than a
              single letter) cannot be printed using this command; use the C
              command for those glyphs.  This command is a groff extension; it
              is only used for devices whose DESC file contains the tcommand
              keyword; see groff_font(5).

       u n xyz...<white-space>
              Print word with track kerning.  This is the same as the t
              command except that after printing each glyph, the current
              horizontal position is increased by the sum of the width of that
              glyph and n (an integer in basic units u).  This command is a
              groff extension; it is only used for devices whose DESC file
              contains the tcommand keyword; see groff_font(5).

       V n    Move down to the absolute vertical position n (a non-negative
              integer in basic units u) relative to upper edge of current
              page.

       v n    Move n basic units u down (n is a non-negative integer).
              [CSTR #54] allows negative values for n also, but groff doesn't
              use this.

       w      Informs about a paddable whitespace to increase readability.
              The spacing itself must be performed explicitly by a move
              command.

   Graphics Commands
       Each graphics or drawing command in the intermediate output starts with
       the letter D followed by one or two characters that specify a
       subcommand; this is followed by a fixed or variable number of integer
       arguments that are separated by a single space character.  A D command
       may not be followed by another command on the same line (apart from a
       comment), so each D command is terminated by a syntactical line break.

       troff output follows the classical spacing rules (no space between
       command and subcommand, all arguments are preceded by a single space
       character), but the parser allows optional space between the command
       letters and makes the space before the first argument optional.  As
       usual, each space can be any sequence of tab and space characters.

       Some graphics commands can take a variable number of arguments.  In
       this case, they are integers representing a size measured in basic
       units u.  The h arguments stand for horizontal distances where positive
       means right, negative left.  The v arguments stand for vertical
       distances where positive means down, negative up.  All these distances
       are offsets relative to the current location.

       Unless indicated otherwise, each graphics command directly corresponds
       to a similar groff \D escape sequence; see groff(7).

       Unknown D commands are assumed to be device-specific.  Its arguments
       are parsed as strings; the whole information is then sent to the
       postprocessor.

       In the following command reference, the syntax element <line-break>
       means a syntactical line break as defined in section Separation.

       D~ h1 v1  h2 v2 ... hn vn<line-break>
              Draw B-spline from current position to offset (h1, v1), then to
              offset (h2, v2) if given, etc., up to (hn, vn).  This command
              takes a variable number of argument pairs; the current position
              is moved to the terminal point of the drawn curve.

       Da h1 v1  h2 v2<line-break>
              Draw arc from current position to (h1, v1)+(h2, v2) with center
              at (h1, v1); then move the current position to the final point
              of the arc.

       DC d<line-break>
       DC d dummy-arg<line-break>
              Draw a solid circle using the current fill color with diameter d
              (integer in basic units u) with leftmost point at the current
              position; then move the current position to the rightmost point
              of the circle.  An optional second integer argument is ignored
              (this allows to the formatter to generate an even number of
              arguments).  This command is a groff extension.

       Dc d<line-break>
              Draw circle line with diameter d (integer in basic units u) with
              leftmost point at the current position; then move the current
              position to the rightmost point of the circle.

       DE h v<line-break>
              Draw a solid ellipse in the current fill color with a horizontal
              diameter of h and a vertical diameter of v (both integers in
              basic units u) with the leftmost point at the current position;
              then move to the rightmost point of the ellipse.  This command
              is a groff extension.

       De h v<line-break>
              Draw an outlined ellipse with a horizontal diameter of h and a
              vertical diameter of v (both integers in basic units u) with the
              leftmost point at current position; then move to the rightmost
              point of the ellipse.

       DF color-scheme [component ...]<line-break>
              Set fill color for solid drawing objects using different color
              schemes; the analoguous command for setting the color of text,
              line graphics, and the outline of graphic objects is m.  The
              color components are specified as integer arguments between 0
              and 65536.  The number of color components and their meaning
              vary for the different color schemes.  These commands are
              generated by the groff escape sequences \D'F ...'  and \M (with
              no other corresponding graphics commands).  No position
              changing.  This command is a groff extension.

              DFc cyan magenta yellow<line-break>
                     Set fill color for solid drawing objects using the CMY
                     color scheme, having the 3 color components cyan,
                     magenta, and yellow.

              DFd <line-break>
                     Set fill color for solid drawing objects to the default
                     fill color value (black in most cases).  No component
                     arguments.

              DFg gray<line-break>
                     Set fill color for solid drawing objects to the shade of
                     gray given by the argument, an integer between 0 (black)
                     and 65536 (white).

              DFk cyan magenta yellow black<line-break>
                     Set fill color for solid drawing objects using the CMYK
                     color scheme, having the 4 color components cyan,
                     magenta, yellow, and black.

              DFr red green blue<line-break>
                     Set fill color for solid drawing objects using the RGB
                     color scheme, having the 3 color components red, green,
                     and blue.

       Df n<line-break>
              The argument n must be an integer in the range -32767 to 32767.

              0<=n<=1000
                     Set the color for filling solid drawing objects to a
                     shade of gray, where 0 corresponds to solid white, 1000
                     (the default) to solid black, and values inbetween to
                     intermediate shades of gray; this is obsoleted by command
                     DFg.

              n<0 or n>1000
                     Set the filling color to the color that is currently
                     being used for the text and the outline, see command m.
                     For example, the command sequence

                            mg 0 0 65536
                            Df -1

                     sets all colors to blue.

              No position changing.  This command is a groff extension.

       Dl h v<line-break>
              Draw line from current position to offset (h, v) (integers in
              basic units u); then set current position to the end of the
              drawn line.

       Dp h1 v1  h2 v2 ... hn vn<line-break>
              Draw a polygon line from current position to offset (h1, v1),
              from there to offset (h2, v2), etc., up to offset (hn, vn), and
              from there back to the starting position.  For historical
              reasons, the position is changed by adding the sum of all
              arguments with odd index to the actual horizontal position and
              the even ones to the vertical position.  Although this doesn't
              make sense it is kept for compatibility.  This command is a
              groff extension.

       DP h1 v1  h2 v2 ... hn vn<line-break>
              The same macro as the corresponding Dp command with the same
              arguments, but draws a solid polygon in the current fill color
              rather than an outlined polygon.  The position is changed in the
              same way as with Dp.  This command is a groff extension.

       Dt n<line-break>
              Set the current line thickness to n (an integer in basic
              units u) if n>0; if n=0 select the smallest available line
              thickness; if n<0 set the line thickness proportional to the
              point size (this is the default before the first Dt command was
              specified).  For historical reasons, the horizontal position is
              changed by adding the argument to the actual horizontal
              position, while the vertical position is not changed.  Although
              this doesn't make sense it is kept for compatibility.  This
              command is a groff extension.

   Device Control Commands
       Each device control command starts with the letter x followed by a
       space character (optional or arbitrary space/tab in groff) and a
       subcommand letter or word; each argument (if any) must be preceded by a
       syntactical space.  All x commands are terminated by a syntactical line
       break; no device control command can be followed by another command on
       the same line (except a comment).

       The subcommand is basically a single letter, but to increase
       readability, it can be written as a word, i.e., an arbitrary sequence
       of characters terminated by the next tab, space, or newline character.
       All characters of the subcommand word but the first are simply ignored.
       For example, troff outputs the initialization command x i as x init and
       the resolution command x r as x res.  But writings like x i_like_groff
       and x roff_is_groff are accepted as well to mean the same commands.

       In the following, the syntax element <line-break> means a syntactical
       line break as defined in section Separation.

       xF name<line-break>
              (Filename control command)
              Use name as the intended name for the current file in error
              reports.  This is useful for remembering the original file name
              when groff uses an internal piping mechanism.  The input file is
              not changed by this command.  This command is a groff extension.

       xf n s<line-break>
              (font control command)
              Mount font position n (a non-negative integer) with font named s
              (a text word), cf.  groff_font(5).

       xH n<line-break>
              (Height control command)
              Set character height to n (a positive integer in scaled
              points z).  Classical troff used the unit points (p) instead;
              see section COMPATIBILITY.

       xi <line-break>
              (init control command)
              Initialize device.  This is the third command of the prologue.

       xp <line-break>
              (pause control command)
              Parsed but ignored.  The classical documentation reads pause
              device, can be restarted.

       xr n h v<line-break>
              (resolution control command)
              Resolution is n, while h is the minimal horizontal motion, and v
              the minimal vertical motion possible with this device; all
              arguments are positive integers in basic units u per inch.  This
              is the second command of the prologue.

       xS n<line-break>
              (Slant control command)
              Set slant to n degrees (an integer in basic units u).

       xs <line-break>
              (stop control command)
              Terminates the processing of the current file; issued as the
              last command of any intermediate troff output.

       xt <line-break>
              (trailer control command)
              Generate trailer information, if any.  In groff, this is
              actually just ignored.

       xT xxx<line-break>
              (Typesetter control command)
              Set name of device to word xxx, a sequence of characters ended
              by the next whitespace character.  The possible device names
              coincide with those from the groff -T option.  This is the first
              command of the prologue.

       xu n<line-break>
              (underline control command)
              Configure underlining of spaces.  If n is 1, start underlining
              of spaces; if n is 0, stop underlining of spaces.  This is
              needed for the cu request in nroff mode and is ignored
              otherwise.  This command is a groff extension.

       xX anything<line-break>
              (X-escape control command)
              Send string anything uninterpreted to the device.  If the line
              following this command starts with a + character this line is
              interpreted as a continuation line in the following sense.  The
              + is ignored, but a newline character is sent instead to the
              device, the rest of the line is sent uninterpreted.  The same
              applies to all following lines until the first character of a
              line is not a + character.  This command is generated by the
              groff escape sequence \X.  The line-continuing feature is a
              groff extension.

   Obsolete Command
       In classical troff output, emitting a single glyph was mostly done by a
       very strange command that combined a horizontal move and the printing
       of a glyph.  It didn't have a command code, but is represented by a
       3-character argument consisting of exactly 2 digits and a character.

       ddc    Move right dd (exactly two decimal digits) basic units u, then
              print glyph with single-letter name c.

              In groff, arbitrary syntactical space around and within this
              command is allowed to be added.  Only when a preceding command
              on the same line ends with an argument of variable length a
              separating space is obligatory.  In classical troff, large
              clusters of these and other commands were used, mostly without
              spaces; this made such output almost unreadable.

       For modern high-resolution devices, this command does not make sense
       because the width of the glyphs can become much larger than two decimal
       digits.  In groff, this is only used for the devices X75, X75-12, X100,
       and X100-12.  For other devices, the commands t and u provide a better
       functionality.

POSTPROCESSING
       The roff postprocessors are programs that have the task to translate
       the intermediate output into actions that are sent to a device.  A
       device can be some piece of hardware such as a printer, or a software
       file format suitable for graphical or text processing.  The groff
       system provides powerful means that make the programming of such
       postprocessors an easy task.

       There is a library function that parses the intermediate output and
       sends the information obtained to the device via methods of a class
       with a common interface for each device.  So a groff postprocessor must
       only redefine the methods of this class.  For details, see the
       reference in section FILES.

EXAMPLES
       This section presents the intermediate output generated from the same
       input for three different devices.  The input is the sentence hell
       world fed into groff on the command line.

       o High-resolution device ps

         shell> echo "hell world" | groff -Z -T ps

         x T ps
         x res 72000 1 1
         x init
         p1
         x font 5 TR
         f5
         s10000
         V12000
         H72000
         thell
         wh2500
         tw
         H96620
         torld
         n12000 0
         x trailer
         V792000
         x stop

       This output can be fed into the postprocessor grops(1) to get its
       representation as a PostScript file.

       o Low-resolution device latin1

         This is similar to the high-resolution device except that the
         positioning is done at a minor scale.  Some comments (lines starting
         with #) were added for clarification; they were not generated by the
         formatter.

         shell> "hell world" | groff -Z -T latin1

         # prologue
         x T latin1
         x res 240 24 40
         x init
         # begin a new page
         p1
         # font setup
         x font 1 R
         f1
         s10
         # initial positioning on the page
         V40
         H0
         # write text `hell'
         thell
         # inform about a space, and do it by a horizontal jump
         wh24
         # write text `world'
         tworld
         # announce line break, but do nothing because ...
         n40 0
         # ... the end of the document has been reached
         x trailer
         V2640
         x stop

       This output can be fed into the postprocessor grotty(1) to get a
       formatted text document.

       o Classical style output

         As a computer monitor has a very low resolution compared to modern
         printers the intermediate output for the X devices can use the jump-
         and-write command with its 2-digit displacements.

         shell> "hell world" | groff -Z -T X100

         x T X100
         x res 100 1 1
         x init
         p1
         x font 5 TR
         f5
         s10
         V16
         H100
         # write text with old-style jump-and-write command
         ch07e07l03lw06w11o07r05l03dh7
         n16 0
         x trailer
         V1100
         x stop

       This output can be fed into the postprocessor xditview(1x) or
       gxditview(1) for displaying in X.

       Due to the obsolete jump-and-write command, the text clusters in the
       classical output are almost unreadable.

COMPATIBILITY
       The intermediate output language of the classical troff was first
       documented in [CSTR #97].  The groff intermediate output format is
       compatible with this specification except for the following features.

       o The classical quasi device independence is not yet implemented.

       o The old hardware was very different from what we use today.  So the
         groff devices are also fundamentally different from the ones in
         classical troff.  For example, the classical PostScript device was
         called post and had a resolution of 720 units per inch, while groff's
         ps device has a resolution of 72000 units per inch.  Maybe, by
         implementing some rescaling mechanism similar to the classical quasi
         device independence, these could be integrated into modern groff.

       o The B-spline command D~ is correctly handled by the intermediate
         output parser, but the drawing routines aren't implemented in some of
         the postprocessor programs.

       o The argument of the commands s and x H has the implicit unit scaled
         point z in groff, while classical troff had point (p).  This isn't an
         incompatibility, but a compatible extension, for both units coincide
         for all devices without a sizescale parameter, including all
         classical and the groff text devices.  The few groff devices with a
         sizescale parameter either did not exist, had a different name, or
         seem to have had a different resolution.  So conflicts with classical
         devices are very unlikely.

       o The position changing after the commands Dp, DP, and Dt is illogical,
         but as old versions of groff used this feature it is kept for
         compatibility reasons.

       The differences between groff and classical troff are documented in
       groff_diff(7).

FILES
       /usr/local/share/groff/1.21/font/devname/DESC
              Device description file for device name.

       <groff-source-dir>/src/libs/libdriver/input.cpp
              Defines the parser and postprocessor for the intermediate
              output.  It is located relative to the top directory of the
              groff source tree.  This parser is the definitive specification
              of the groff intermediate output format.

SEE ALSO
       A reference like groff(7) refers to a manual page; here groff in
       section 7 of the man-page documentation system.  To read the example,
       look up section 7 in your desktop help system or call from the shell
       prompt

              shell> man 7 groff

       For more details, see man(1).

       groff(1)
              option -Z and further readings on groff.

       groff(7)
              for details of the groff language such as numerical units and
              escape sequences.

       groff_font(5)
              for details on the device scaling parameters of the DESC file.

       troff(1)
              generates the device-independent intermediate output.

       roff(7)
              for historical aspects and the general structure of roff
              systems.

       groff_diff(7)
              The differences between the intermediate output in groff and
              classical troff.

       gxditview(1)
              Viewer for the intermediate output.

       grodvi(1), grohtml(1), grolbp(1), grolj4(1), grops(1), grotty(1)
              the groff postprocessor programs.

       For a treatment of all aspects of the groff system within a single
       document, see the groff info file.  It can be read within the
       integrated help systems, within emacs(1) or from the shell prompt by
              shell> info groff

       The classical troff output language is described in two AT&T Bell Labs
       CSTR documents available on-line at Bell Labs CSTR site <http://
       cm.bell-labs.com/cm/cs/cstr.html>.

       [CSTR #97]
              A Typesetter-independent TROFF by Brian Kernighan is the
              original and most comprehensive documentation on the output
              language; see CSTR #97 <http://cm.bell-labs.com/cm/cs/cstr/
              97.ps.gz>.

       [CSTR #54]
              The 1992 revision of the Nroff/Troff User's Manual by J. F.
              Ossanna and Brian Kernighan isn't as comprehensive as [CSTR #97]
              regarding the output language; see CSTR #54 <http://cm.bell-
              labs.com/cm/cs/cstr/54.ps.gz>.

AUTHORS
       Copyright (C) 1989, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008,
       2009 Free Software Foundation, Inc.

       This document is distributed under the terms of the FDL (GNU Free
       Documentation License) version 1.3 or later.  You should have received
       a copy of the FDL with this package; it is also available on-line at
       the GNU copyleft site <http://www.gnu.org/copyleft/fdl.html>.

       This document is part of groff, the GNU roff distribution.  It is based
       on a former version - published under the GPL - that described only
       parts of the groff extensions of the output language.  It was rewritten
       in 2002 by Bernd Warken and is maintained by Werner Lemberg
       <wl@gnu.org>.



Groff Version 1.21             31 December 2010                   GROFF_OUT(5)
