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(groff)Tabs and Fields

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Tabs and Fields
===============

   A tab character (ASCII char 9, EBCDIC char 5) causes a horizontal
movement to the next tab stop (much like it did on a typewriter).

 - Escape: \t
     This escape is a non-interpreted tab character.  In copy mode
     (Note: Copy-in Mode), `\t' is the same as a real tab character.

 - Request: .ta [n1 n2 ... nn T r1 r2 ... rn]
 - Register: \n[.tabs]
     Change tab stop positions.  This request takes a series of tab
     specifiers as arguments (optionally divided into two groups with
     the letter `T') which indicate where each tab stop is to be
     (overriding any previous settings).

     Tab stops can be specified absolutely, i.e., as the distance from
     the left margin.  For example, the following sets 6 tab stops every
     one inch.


          .ta 1i 2i 3i 4i 5i 6i

     Tab stops can also be specified using a leading `+' which means
     that the specified tab stop is set relative to the previous tab
     stop.  For example, the following is equivalent to the previous
     example.


          .ta 1i +1i +1i +1i +1i +1i

     `gtroff' supports an extended syntax to specify repeat values after
     the `T' mark (these values are always taken as relative) - this is
     the usual way to specify tabs set at equal intervals.  The
     following is, yet again, the same as the previous examples.  It
     does even more since it defines an infinite number of tab stops
     separated by one inch.


          .ta T 1i

     Now we are ready to interpret the full syntax given at the
     beginning: Set tabs at positions N1, N2, ..., NN and then set tabs
     at NN+R1, NN+R2, ..., NN+RN and then at NN+RN+R1, NN+RN+R2, ...,
     NN+RN+RN, and so on.

     Example: `4c +6c T 3c 5c 2c' is equivalent to `4c 10c 13c 18c 20c
     23c 28c 30c ...'.

     The material in each tab column (i.e., the column between two tab
     stops) may be justified to the right or left or centered in the
     column.  This is specified by appending `R', `L', or `C' to the tab
     specifier.  The default justification is `L'.  Example:


          .ta 1i 2iC 2iR

     Some notes:

        * The default unit of the `ta' request is `m'.

        * A tab stop is converted into a non-breakable horizontal
          movement which can be neither stretched nor squeezed.  For
          example,


               .ds foo a\tb\tc
               .ta T 5i
               \*[foo]

          creates a single line which is a bit longer than 10 inches (a
          string is used to show exactly where the tab characters are).
          Now consider the following:


               .ds bar a\tb b\tc
               .ta T 5i
               \*[bar]

          `gtroff' first converts the tab stops of the line into
          unbreakable horizontal movements, then splits the line after
          the second `b' (assuming a sufficiently short line length).
          Usually, this isn't what the user wants.

        * Superfluous tabs (i.e., tab characters which do not
          correspond to a tab stop) are ignored except the first one
          which delimits the characters belonging to the last tab stop
          for right-justifying or centering.  Consider the following
          example


               .ds Z   foo\tbar\tfoo
               .ds ZZ  foo\tbar\tfoobar
               .ds ZZZ foo\tbar\tfoo\tbar
               .ta 2i 4iR
               \*[Z]
               .br
               \*[ZZ]
               .br
               \*[ZZZ]
               .br

          which produces the following output:


               foo                 bar              foo
               foo                 bar           foobar
               foo                 bar              foobar

          The first line right-justifies the second `foo' relative to
          the tab stop.  The second line right-justifies `foobar'.  The
          third line finally right-justifies only `foo' because of the
          additional tab character which marks the end of the string
          belonging to the last defined tab stop.

        * Tab stops are associated with the current environment (Note:
          Environments).

        * Calling `ta' without an argument removes all tab stops.

        * The start-up value of `gtroff' is `T 0.5i'.  This value is
          used even for tty output devices (contrary to UNIX `nroff'
          which has tab stops preset every 0.8i).


     The read-only number register `.tabs' contains a string
     representation of the current tab settings suitable for use as an
     argument to the `ta' request.


          .ds tab-string \n[.tabs]
          \*[tab-string]
              => T120u


 - Request: .tc [fill-char]
     Normally `gtroff' fills the space to the next tab stop with
     whitespace.  This can be changed with the `tc' request.  With no
     argument `gtroff' reverts to using whitespace, which is the
     default.  The value of this "tab repetition" character is
     associated with the current environment (Note: Environments).

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