Indentation
***********

Indentation refers to inserting or adjusting 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.  See Program Indent, for additional documentation about indenting in programming modes.

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

TAB
     Insert whitespace, or indent the current line, in a mode-appropriate way (indent-for-tab-command).  If the region is active, indent all the lines within it.

The exact behavior of TAB depends on the major mode.  In Text mode and related major modes, TAB normally inserts some combination of space and tab characters to advance point to the next tab stop (see 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 TAB to ``align'' point with the preceding line.  If the region is active (see Using Region), 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, 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 C-q TAB (see Inserting Text).

Indentation Commands
====================

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

C-j
     Perform RET followed by TAB (newline-and-indent).

C-M-o
     Split the current line at point (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.

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

M-i
     Indent whitespace at point, up to the next tab stop (tab-to-tab-stop).  See Tab Stops.

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 tab-to-tab-stop instead---unless called with a numeric argument, in which case do nothing.

M-^
     Merge the previous and the current line (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, M-^ deletes the fill prefix if it appears after the newline that is deleted.  See Fill Prefix.

C-M-\
     Indent all the lines in the region, as though you had typed TAB at the beginning of each line (indent-region).

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

C-x TAB
     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 LEFT or RIGHT indents leftward and rightward, respectively, by one space.  You can also type S-LEFT or S-RIGHT to indent leftward or rightward to the next tab stop (see Tab Stops).  Typing any other key disables the transient mode, and resumes normal editing.

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

          C-u -999 C-x TAB

Tab Stops
=========

Emacs defines certain column numbers to be tab stops.  These are used as stopping points by TAB when inserting whitespace in Text mode and related modes (see Indentation), and by commands like M-i (see Indentation Commands).  By default, tab stops are located every 8 columns.  These positions are stored in the variable tab-stop-list, whose value is a list of column numbers in increasing order.

Instead of customizing the variable tab-stop-list directly, a convenient way to view and set tab stops is via the command M-x edit-tab-stops.  This switches to a buffer containing a description of the tab stop settings, which looks like this:

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

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.  It is implicitly extended to infinity by repeating the last step.

You can edit this buffer to specify different tab stops by placing colons on the desired columns.  The buffer uses Overwrite mode (see Minor Modes).  When you are done, type 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 tab-stop-list variable local to the buffer where you called M-x edit-tab-stops (see 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 tab-stop-list (see 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 display tab stop.  See Text Display.

Tabs vs. Spaces
===============

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 display tab stop.  By default, there is one display tab stop every tab-width columns (the default is 8).  See Text Display.

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

One reason to set indent-tabs-mode to nil is that not all editors display tab characters in the same way.  Emacs users, too, may have different customized values of 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 tab-width variable in a file-local variable (see File Variables).

There are also commands to convert tabs to spaces or vice versa, always preserving the columns of all non-whitespace text.  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.  M-x untabify changes all tabs in the region to appropriate numbers of spaces.

Convenience Features for Indentation
====================================

The variable tab-always-indent tweaks the behavior of the TAB (indent-for-tab-command) command.  The default value, t, gives the behavior described in Indentation.  If you change the value to the symbol complete, then TAB first tries to indent the current line, and if the line was already indented, it tries to complete the text at point (see Symbol Completion).  If the value is nil, then 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.

Electric Indent mode is a global minor mode that automatically indents the line after every RET you type.  This mode is enabled by default.  To toggle this minor mode, type M-x electric-indent-mode.