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

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

TAB
     Indent current line ``appropriately'' in a mode-dependent fashion.

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

M-^
     Merge two lines (delete-indentation).  This would cancel out the effect of LFD.

C-M-o
     Split line at point; text on the line after point becomes a new line indented to the same column that it now starts in (split-line).

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

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

C-x TAB
     Shift block of lines 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.

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.

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

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

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

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

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.

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 LFD.

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.  C-M-\ (indent-region) gives each line which begins in the region the ``usual'' indentation by invoking 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.

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, indent-relative runs tab-to-tab-stop (see Tab Stops).

indent-relative is the definition of TAB in Indented Text mode.  See Text.

Tab Stops
=========

For typing in tables, you can use Text mode's definition of TAB, 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 M-i.

You can specify the tab stops used by M-i.  They are stored in a variable called tab-stop-list, as a list of column-numbers in increasing order.

The 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.  In the tab stop buffer, C-c C-c runs the function edit-tab-stops-note-changes rather than its usual definition save-buffer.  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 displaying tab characters in the buffer.  See Display Vars, for more information on that.

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; altering the variable affects only the current buffer, but there is a default value which you can change as well.  See Locals.

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 three 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.