@c This is part of the Emacs manual.
@c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Indentation, Text, Major Modes, Top
@chapter Indentation
@cindex indentation
@cindex columns (indentation)

  This chapter describes the Emacs commands that add, remove, or
adjust indentation.

@table @kbd
@item @key{TAB}
Indent the current line ``appropriately'' in a mode-dependent fashion.
@item @kbd{C-j}
Perform @key{RET} followed by @key{TAB} (@code{newline-and-indent}).
@item M-^
Merge the previous and the current line (@code{delete-indentation}).
This would cancel out the effect of @kbd{C-j}.
@item C-M-o
Split the current line at point; text on the line after point becomes a
new line indented to the same column where point is located
(@code{split-line}).
@item M-m
Move (forward or back) to the first nonblank character on the current
line (@code{back-to-indentation}).
@item C-M-\
Indent several lines to the same column (@code{indent-region}).
@item C-x @key{TAB}
Shift a block of lines rigidly right or left (@code{indent-rigidly}).
@item M-i
Indent from point to the next prespecified tab stop column
(@code{tab-to-tab-stop}).
@item M-x indent-relative
Indent from point to under an indentation point in the previous line.
@end table

  Most programming languages have some indentation convention.  For Lisp
code, lines are indented according to their nesting in parentheses.  The
same general idea is used for C code, though many details are different.

@kindex TAB
  Whatever the language, to indent a line, use the @key{TAB} command.  Each
major mode defines this command to perform the sort of indentation
appropriate for the particular language.  In Lisp mode, @key{TAB} aligns
the line according to its depth in parentheses.  No matter where in the
line you are when you type @key{TAB}, it aligns the line as a whole.  In C
mode, @key{TAB} implements a subtle and sophisticated indentation style that
knows about many aspects of C syntax.

  In Text mode, @key{TAB} runs the command @code{tab-to-tab-stop}, which
indents to the next tab stop column.  You can set the tab stops with
@kbd{M-x edit-tab-stops}.

  Normally, @key{TAB} inserts an optimal mix of tabs and spaces for
the intended indentation.  @xref{Just Spaces}, for how to prevent use
of tabs.

@menu
* Indentation Commands::  Various commands and techniques for indentation.
* Tab Stops::             You can set arbitrary "tab stops" and then
                            indent to the next tab stop when you want to.
* Just Spaces::           You can request indentation using just spaces.
@end menu

@node Indentation Commands, Tab Stops, Indentation, Indentation
@section Indentation Commands and Techniques

@kindex M-m
@findex back-to-indentation
  To move over the indentation on a line, do @kbd{M-m}
(@code{back-to-indentation}).  This command, given anywhere on a line,
positions point at the first nonblank character on the line.

  To insert an indented line before the current line, do @kbd{C-a C-o
@key{TAB}}.  To make an indented line after the current line, use
@kbd{C-e C-j}.

  If you just want to insert a tab character in the buffer, you can type
@kbd{C-q @key{TAB}}.

@kindex C-M-o
@findex split-line
  @kbd{C-M-o} (@code{split-line}) moves the text from point to the end of
the line vertically down, so that the current line becomes two lines.
@kbd{C-M-o} first moves point forward over any spaces and tabs.  Then it
inserts after point a newline and enough indentation to reach the same
column point is on.  Point remains before the inserted newline; in this
regard, @kbd{C-M-o} resembles @kbd{C-o}.

@kindex M-^
@findex delete-indentation
  To join two lines cleanly, use the @kbd{M-^}
(@code{delete-indentation}) command.  It deletes the indentation at the
front of the current line, and the line boundary as well, replacing them
with a single space.  As a special case (useful for Lisp code) the
single space is omitted if the characters to be joined are consecutive
open parentheses or closing parentheses, or if the junction follows
another newline.  To delete just the indentation of a line, go to the
beginning of the line and use @kbd{M-\}
(@code{delete-horizontal-space}), which deletes all spaces and tabs
around the cursor.

  If you have a fill prefix, @kbd{M-^} deletes the fill prefix if it
appears after the newline that is deleted.  @xref{Fill Prefix}.

@kindex C-M-\
@kindex C-x TAB
@findex indent-region
@findex indent-rigidly
  There are also commands for changing the indentation of several lines
at once.  @kbd{C-M-\} (@code{indent-region}) applies to all the lines
that begin in the region; it indents each line in the ``usual'' way, as
if you had typed @key{TAB} at the beginning of the line.  A numeric
argument specifies the column to indent to, and each line is shifted
left or right so that its first nonblank character appears in that
column.  @kbd{C-x @key{TAB}} (@code{indent-rigidly}) moves all of the
lines in the region right by its argument (left, for negative
arguments).  The whole group of lines moves rigidly sideways, which is
how the command gets its name.@refill

@findex indent-relative
  @kbd{M-x indent-relative} indents at point based on the previous line
(actually, the last nonempty line).  It inserts whitespace at point, moving
point, until it is underneath an indentation point in the previous line.
An indentation point is the end of a sequence of whitespace or the end of
the line.  If point is farther right than any indentation point in the
previous line, the whitespace before point is deleted and the first
indentation point then applicable is used.  If no indentation point is
applicable even then, @code{indent-relative} runs @code{tab-to-tab-stop}
@ifinfo
(@pxref{Tab Stops}),
@end ifinfo
@iftex
(see next section),
@end iftex
unless it is called with a numeric argument, in which case it does
nothing.

  @code{indent-relative} is the definition of @key{TAB} in Indented Text
mode.  @xref{Text}.

  @xref{Format Indentation}, for another way of specifying the
indentation for part of your text.

@node Tab Stops, Just Spaces, Indentation Commands, Indentation
@section Tab Stops

@cindex tab stops 
@cindex using tab stops in making tables
@cindex tables, indentation for
@kindex M-i
@findex tab-to-tab-stop
  For typing in tables, you can use Text mode's definition of @key{TAB},
@code{tab-to-tab-stop}.  This command inserts indentation before point,
enough to reach the next tab stop column.  If you are not in Text mode,
this command can be found on the key @kbd{M-i}.

@findex edit-tab-stops
@findex edit-tab-stops-note-changes
@kindex C-c C-c @r{(Edit Tab Stops)}
@vindex tab-stop-list
  You can specify the tab stops used by @kbd{M-i}.  They are stored in a
variable called @code{tab-stop-list}, as a list of column-numbers in
increasing order.

  The convenient way to set the tab stops is with @kbd{M-x
edit-tab-stops}, which creates and selects a buffer containing a
description of the tab stop settings.  You can edit this buffer to
specify different tab stops, and then type @kbd{C-c C-c} to make those
new tab stops take effect.  @code{edit-tab-stops} records which buffer
was current when you invoked it, and stores the tab stops back in that
buffer; normally all buffers share the same tab stops and changing them
in one buffer affects all, but if you happen to make
@code{tab-stop-list} local in one buffer then @code{edit-tab-stops} in
that buffer will edit the local settings.

  Here is what the text representing the tab stops looks like for ordinary
tab stops every eight columns.

@example
        :       :       :       :       :       :
0         1         2         3         4
0123456789012345678901234567890123456789012345678
To install changes, type C-c C-c
@end example

  The first line contains a colon at each tab stop.  The remaining lines
are present just to help you see where the colons are and know what to do.

  Note that the tab stops that control @code{tab-to-tab-stop} have nothing
to do with displaying tab characters in the buffer.  @xref{Display Custom},
for more information on that.

@node Just Spaces,, Tab Stops, Indentation
@section Tabs vs. Spaces

@vindex indent-tabs-mode
  Emacs normally uses both tabs and spaces to indent lines.  If you
prefer, all indentation can be made from spaces only.  To request
this, set @code{indent-tabs-mode} to @code{nil}.  This is a per-buffer
variable, so altering the variable affects only the current buffer,
but there is a default value which you can change as well.
@xref{Locals}.

@findex tabify
@findex untabify
  There are also commands to convert tabs to spaces or vice versa, always
preserving the columns of all nonblank text.  @kbd{M-x tabify} scans the
region for sequences of spaces, and converts sequences of at least three
spaces to tabs if that can be done without changing indentation.  @kbd{M-x
untabify} changes all tabs in the region to appropriate numbers of spaces.
