@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2001-2016 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node Indentation
@chapter Indentation
@cindex indentation
@cindex tabs
@cindex columns (indentation)

@cindex whitespace character
  @dfn{Indentation} refers to inserting or adjusting @dfn{whitespace
characters} (space and/or tab characters) at the beginning of a line
of text.  This chapter documents indentation commands and options
which are common to Text mode and related modes, as well as
programming language modes.  @xref{Program Indent}, for additional
documentation about indenting in programming modes.

@findex indent-for-tab-command
@kindex TAB @r{(indentation)}
  The simplest way to perform indentation is the @key{TAB} key.  In
most major modes, this runs the command @code{indent-for-tab-command}.
(In C and related modes, @key{TAB} runs the command
@code{c-indent-line-or-region}, which behaves similarly).

@table @key
@item TAB
Insert whitespace, or indent the current line, in a mode-appropriate
way (@code{indent-for-tab-command}).  If the region is active, indent
all the lines within it.
@end table

  The exact behavior of @key{TAB} depends on the major mode.  In Text
mode and related major modes, @key{TAB} normally inserts some
combination of space and tab characters to advance point to the next
tab stop (@pxref{Tab Stops}).  For this purpose, the position of the
first non-whitespace character on the preceding line is treated as an
additional tab stop, so you can use @key{TAB} to align point with
the preceding line.  If the region is active (@pxref{Using Region}),
@key{TAB} acts specially: it indents each line in the region so that
its first non-whitespace character is aligned with the preceding line.

  In programming modes, @key{TAB} indents the current line of code in
a way that makes sense given the code in the preceding lines.  If the
region is active, all the lines in the region are indented this way.
If point was initially within the current line's indentation, it is
repositioned to the first non-whitespace character on the line.

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

@menu
* Indentation Commands::  More commands for performing indentation.
* Tab Stops::             Stop points for indentation in Text modes.
* Just Spaces::           Using only space characters for indentation.
* Indent Convenience::    Optional indentation features.
@end menu

@node Indentation Commands
@section Indentation Commands

Apart from the @key{TAB} (@code{indent-for-tab-command}) command,
Emacs provides a variety of commands to perform indentation in other
ways.

@table @kbd
@item C-M-o
@kindex C-M-o
@findex split-line
Split the current line at point (@code{split-line}).  The text on the
line after point becomes a new line, indented to the same column where
point is located.  This command first moves point forward over any
spaces and tabs.  Afterward, point is positioned before the inserted
newline.

@kindex M-m
@findex back-to-indentation
@item M-m
Move (forward or back) to the first non-whitespace character on the
current line (@code{back-to-indentation}).  If there are no
non-whitespace characters on the line, move to the end of the line.

@item M-i
@kindex M-i
@findex tab-to-tab-stop
Indent whitespace at point, up to the next tab stop
(@code{tab-to-tab-stop}).  @xref{Tab Stops}.

@findex indent-relative
@item M-x indent-relative
Insert whitespace at point, until point is aligned with the first
non-whitespace character on the previous line (actually, the last
non-blank line).  If point is already farther right than that, run
@code{tab-to-tab-stop} instead---unless called with a numeric
argument, in which case do nothing.

@item M-^
@kindex M-^
@findex delete-indentation
Merge the previous and the current line (@code{delete-indentation}).
This joins the two lines cleanly, by replacing any indentation at
the front of the current line, together with the line boundary, 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 opening and closing
parentheses, or if the junction follows another newline.

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

@item C-M-\
@kindex C-M-\
@findex indent-region
Indent all the lines in the region, as though you had typed @key{TAB}
at the beginning of each line (@code{indent-region}).

If a numeric argument is supplied, indent every line in the region to
that column number.

@item C-x @key{TAB}
@kindex C-x TAB
@findex indent-rigidly
@cindex remove indentation
This command is used to change the indentation of all lines that begin
in the region, moving the affected lines as a rigid unit.

If called with no argument, the command activates a transient mode for
adjusting the indentation of the affected lines interactively.  While
this transient mode is active, typing @key{LEFT} or @key{RIGHT}
indents leftward and rightward, respectively, by one space.  You can
also type @kbd{S-@key{LEFT}} or @kbd{S-@key{RIGHT}} to indent leftward
or rightward to the next tab stop (@pxref{Tab Stops}).  Typing any
other key disables the transient mode, and resumes normal editing.

If called with a prefix argument @var{n}, this command indents the
lines forward by @var{n} spaces (without enabling the transient mode).
Negative values of @var{n} indent backward, so you can remove all
indentation from the lines in the region using a large negative
argument, like this:

@smallexample
C-u -999 C-x @key{TAB}
@end smallexample
@end table

@node Tab Stops
@section Tab Stops
@cindex tab stops

@vindex tab-stop-list
  Emacs defines certain column numbers to be @dfn{tab stops}.  These
are used as stopping points by @key{TAB} when inserting whitespace in
Text mode and related modes (@pxref{Indentation}), and by commands
like @kbd{M-i} (@pxref{Indentation Commands}).  The variable
@code{tab-stop-list} controls these positions.  The default value is
@code{nil}, which means a tab stop every 8 columns.  The value can
also be a list of zero-based column numbers (in increasing order) at
which to place tab stops.  Emacs extends the list forever by repeating
the difference between the last and next-to-last elements.

@findex edit-tab-stops
@kindex C-c C-c @r{(Edit Tab Stops)}
  Instead of customizing the variable @code{tab-stop-list} directly, a
convenient way to view and set tab stops is via the command @kbd{M-x
edit-tab-stops}.  This switches to a buffer containing a description
of the tab stop settings, which looks like this:

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

@noindent
The first line contains a colon at each tab stop.  The numbers on the
next two lines are present just to indicate where the colons are.
If the value of @code{tab-stop-list} is @code{nil}, as it is by default,
no colons are displayed initially.

  You can edit this buffer to specify different tab stops by placing
colons on the desired columns.  The buffer uses Overwrite mode
(@pxref{Minor Modes}).  Remember that Emacs will extend the list of
tab stops forever by repeating the difference between the last two
explicit stops that you place.  When you are done, type @kbd{C-c C-c} to make
the new tab stops take effect.  Normally, the new tab stop settings
apply to all buffers.  However, if you have made the
@code{tab-stop-list} variable local to the buffer where you called
@kbd{M-x edit-tab-stops} (@pxref{Locals}), then the new tab stop
settings apply only to that buffer.  To save the tab stop settings for
future Emacs sessions, use the Customize interface to save the value
of @code{tab-stop-list} (@pxref{Easy Customization}).

  Note that the tab stops discussed in this section have nothing to do
with how tab characters are displayed in the buffer.  Tab characters
are always displayed as empty spaces extending to the next
@dfn{display tab stop}.  @xref{Text Display}.

@node Just Spaces
@section Tabs vs.@: Spaces

@vindex tab-width
  Normally, indentation commands insert (or remove) an optimal mix of
space characters and tab characters to align to the desired column.
Tab characters are displayed as a stretch of empty space extending to
the next @dfn{display tab stop}.  By default, there is one display tab
stop every @code{tab-width} columns (the default is 8).  @xref{Text
Display}.

@vindex indent-tabs-mode
  If you prefer, all indentation can be made from spaces only.  To
request this, set the buffer-local variable @code{indent-tabs-mode} to
@code{nil}.  @xref{Locals}, for information about setting buffer-local
variables.  Note, however, that @kbd{C-q @key{TAB}} always inserts a
tab character, regardless of the value of @code{indent-tabs-mode}.

  One reason to set @code{indent-tabs-mode} to @code{nil} is that not
all editors display tab characters in the same way.  Emacs users, too,
may have different customized values of @code{tab-width}.  By using
spaces only, you can make sure that your file always looks the same.
If you only care about how it looks within Emacs, another way to
tackle this problem is to set the @code{tab-width} variable in a
file-local variable (@pxref{File Variables}).

@findex tabify
@findex untabify
  There are also commands to convert tabs to spaces or vice versa, always
preserving the columns of all non-whitespace text.  @kbd{M-x tabify} scans the
region for sequences of spaces, and converts sequences of at least two
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.

@node Indent Convenience
@section Convenience Features for Indentation

@vindex tab-always-indent
  The variable @code{tab-always-indent} tweaks the behavior of the
@key{TAB} (@code{indent-for-tab-command}) command.  The default value,
@code{t}, gives the behavior described in @ref{Indentation}.  If you
change the value to the symbol @code{complete}, then @key{TAB} first
tries to indent the current line, and if the line was already
indented, it tries to complete the text at point (@pxref{Symbol
Completion}).  If the value is @code{nil}, then @key{TAB} indents the
current line only if point is at the left margin or in the line's
indentation; otherwise, it inserts a tab character.

@cindex Electric Indent mode
@cindex mode, Electric Indent
@findex electric-indent-mode
  Electric Indent mode is a global minor mode that automatically
indents the line after every @key{RET} you type.  This mode is enabled
by default.  To toggle this minor mode, type @kbd{M-x
electric-indent-mode}.  To toggle the mode in a single buffer,
use @kbd{M-x electric-indent-local-mode}.