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

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

TAB
     Indent the current line appropriately, in a mode-dependent fashion.

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

M-^
     Merge the previous and the current line (delete-indentation). This would cancel the effect of a preceding C-j.

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 (split-line).

M-m
     Move (forward or back) to the first nonblank character on the current line (back-to-indentation).

C-M-\
     Indent lines in the region to the same column (indent-region).

C-x TAB
     Shift lines in the region rigidly right or left (indent-rigidly).

M-i
     Indent from point to the next prespecified tab stop column (tab-to-tab-stop).

M-x indent-relative
     Indent from point to under an indentation point in the previous line.

In most major modes, the TAB key runs the command indent-for-tab-command, which either performs indentation or inserts whitespace at point, depending on the situation.

In programming modes such as Lisp mode and C mode, TAB indents the current line if the region is inactive.  If the region is active, it indents every line in the region (see Mark).  Indentation means adding or removing some combination of space and tab characters (whitespace characters) at the start of the line, in a way that makes sense given the text in the preceding lines.  Exactly how indentation is performed depends on the major mode.  See Program Indent.

In text modes, TAB inserts some whitespace characters to advance point to the next tab stop (see Tab Stops).  For the purposes of this command, the position of the first non-whitespace character on the preceding line is treated as an additional tab stop. You can therefore use TAB to ``align'' point with the preceding line.  If the region is active, TAB performs this action on every line in the region.

Indentation is often performed with the help of tab characters (ASCII code 9), which are displayed as a stretch of empty space extending to the next display tab stop.  By default, there is one display tab stop every eight columns; the number of columns is determined by the variable tab-width.  You can insert a single tab character by typing C-q TAB.  See Text Display.

The command M-i (tab-to-tab-stop) adjusts the whitespace characters around point, inserting just enough whitespace to advance point up to the next tab stop.  By default, this involves deleting the existing whitespace and inserting a single tab character.

Normally, most of these indentation commands insert an optimal mix of tabs and spaces to align to the desired column.  See Just Spaces, for how to disable use of tabs.  However, C-q TAB always inserts a tab, even when tabs are disabled for the indentation commands.

Indentation Commands and Techniques
===================================

To move over the indentation on a line, do M-m (back-to-indentation).  This command, given anywhere on a line, positions point at the first nonblank character on the line, if any, or else at the end of the line.

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

If you just want to insert a tab character in the buffer, type C-q TAB.

C-M-o (split-line) moves the text from point to the end of the line vertically down, so that the current line becomes two lines. 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, C-M-o resembles C-o.

To join two lines cleanly, use the M-^ (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 M-\ (delete-horizontal-space), which deletes all spaces and tabs around the cursor.

If you have a fill prefix, M-^ deletes the fill prefix if it appears after the newline that is deleted.  See Fill Prefix.

There are also commands for changing the indentation of several lines at once.  They apply to all the lines that begin in the region. C-M-\ (indent-region) indents each line in the ``usual'' way, as if you had typed 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.  C-x TAB (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.

To remove all indentation from all of the lines in the region, invoke C-x TAB with a large negative argument, such as -1000.

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 the next 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, indent-relative runs tab-to-tab-stop (see Tab Stops), unless it is called with a numeric argument, in which case it does nothing.

See Format Indentation, for another way of specifying the indentation for part of your text.

Tab Stops
=========

For typing in tables, you can use M-i (tab-to-tab-stop). This command inserts indentation before point, enough to reach the next tab stop column.

You can change the tab stops used by M-i and other indentation commands, so that they need not be spaced every eight characters, or even regularly spaced.  The tab stops are stored in the variable tab-stop-list, as a list of column numbers in increasing order.

A convenient way to set the tab stops is with 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 C-c C-c to make those new tab stops take effect.  The buffer uses Overwrite mode (see Minor Modes).  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 tab-stop-list local in one buffer then 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.

             :       :       :       :       :       :
     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 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 tab-to-tab-stop 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, which occurs every tab-width columns regardless of the contents of tab-stop-list.  See Text Display.

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

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 indent-tabs-mode to 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. See Locals.

A tab is not always displayed in the same way.  By default, tabs are eight columns wide, but some people like to customize their editors to use a different tab width (e.g., by changing the variable tab-width in Emacs).  By using spaces only, you can make sure that your file looks the same regardless of the tab width setting.

There are also commands to convert tabs to spaces or vice versa, always preserving the columns of all nonblank 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.