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

A tab character (ISO code point 9, EBCDIC code point 5) causes a
horizontal movement to the next tab stop, if any.

 -- Escape sequence: \t
     Interpolate a tab in copy mode; see *note Copy Mode::.

 -- 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') that indicate where each tab stop is to be,
     overriding any previous settings.  The default scaling unit is 'm'.
     Invoking 'ta' without an argument removes all tab stops.  GNU
     'troff''s startup value is 'T 0.5i'.

     Tab stops can be specified absolutely--as distances from the left
     margin.  The following example sets six tab stops, one every 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

     GNU 'troff' supports an extended syntax to specify repeating tab
     stops.  These stops appear after a 'T' argument.  Their values are
     always taken as distances relative to the previous tab stop.  This
     is the idiomatic way to specify tab stops at equal intervals in
     'groff'.  The following is, yet again, the same as the previous
     examples.  It does more, in fact, since it defines an infinite
     number of tab stops at one-inch intervals.

          .ta T 1i

     Now we are ready to interpret the full syntax given above.  The
     'ta' request sets tabs at positions N1, N2, ..., NN, then at NN+R1,
     NN+R2, ..., NN+RN, then at NN+RN+R1, NN+RN+R2, ..., NN+RN+RN, and
     so on.

     For example, '4c +6c T 3c 5c 2c' is equivalent to '4c 10c 13c 18c
     20c 23c 28c 30c ...'.

     Text written to a tab column (i.e., between two tab stops, or
     between a tab stop and an output line boundary) may be aligned to
     the right or left, or centered in the column.  This alignment is
     determined by appending 'R', 'L', or 'C' to the tab specifier.  The
     default is 'L'.

          .ta 1i 2iC 3iR

     The beginning of an output line is not a tab stop; the text that
     begins an output line is placed according to the configured
     alignment and indentation; see *note Manipulating Filling and
     Adjustment:: and *note Line Layout::.

     A tab stop is converted into a non-breakable horizontal movement
     that cannot be adjusted.

          .ll 2i
          .ds foo a\tb\tc
          .ta T 1i
          \*[foo]
              error-> warning: cannot break line
              => a         b         c

     The above creates a single output line that is a bit longer than
     two inches (we use a string to show exactly where the tab stops
     are).  Now consider the following.

          .ll 2i
          .ds bar a\tb c\td
          .ta T 1i
          \*[bar]
              error-> warning: cannot adjust line
              => a         b
              => c       d

     GNU 'troff' first converts the line's tab stops into unbreakable
     horizontal movements, then breaks after 'b'.  This usually isn't
     what you want.

     Superfluous tab characters--those that do not correspond to a tab
     stop--are ignored except for the first, which delimits the
     characters belonging to the last tab stop for right-alignment or
     centering.

          .ds Z   foo\tbar\tbaz
          .ds ZZ  foo\tbar\tbazqux
          .ds ZZZ foo\tbar\tbaz\tqux
          .ta 2i 4iR
          \*[Z]
          .br
          \*[ZZ]
          .br
          \*[ZZZ]
          .br
              => foo                 bar              baz
              => foo                 bar           bazqux
              => foo                 bar              bazqux

     The first line right-aligns "baz" within the second tab stop.  The
     second line right-aligns "bazqux" within it.  The third line
     right-aligns only "baz" because of the additional tab character,
     which marks the end of the text occupying the last tab stop
     defined.

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

     The read-only register '.tabs' contains a string representation of
     the current tab settings suitable for use as an argument to the
     'ta' request.(1)  (*note Tabs and Fields-Footnote-1::)

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

 -- Request: .tc [c]
     Set the tab repetition character to the ordinary or special
     character C; normally, no glyph is written when moving to a tab
     stop (and some output devices may output space characters to
     achieve this motion).  A "tab repetition character" causes the
     formatter to write as many instances of C as are necessary to
     occupy the interval from the horizontal drawing position to the
     next tab stop.  With no argument, GNU 'troff' reverts to the
     default behavior.  The tab repetition character is associated with
     the environment (*note Environments::).  Only a single character of
     C is recognized; any excess is ignored.

 -- Request: .linetabs n
 -- Register: \n[.linetabs]
     If N is missing or non-zero, activate "line-tabs"; deactivate it
     otherwise (the default).  Active line-tabs cause GNU 'troff' to
     compute tab distances relative to the start of the output line
     instead of the input line.

          .de Tabs
          .  ds x a\t\c
          .  ds y b\t\c
          .  ds z c
          .  ta 1i 3i
          \\*x
          \\*y
          \\*z
          ..
          .Tabs
          .br
          .linetabs
          .Tabs
              => a         b         c
              => a         b                   c

     Line-tabs activation is associated with the environment (*note
     Environments::).  The read-only register '.linetabs' interpolates 1
     if line-tabs are active, and 0 otherwise.

* Menu:

* Leaders::
* Fields::