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25.6.2 Explicit Fill Commands

Fill current paragraph (fill-paragraph).
C-x f
Set the fill column (set-fill-column).
M-x fill-region
Fill each paragraph in the region (fill-region).
M-x fill-region-as-paragraph
Fill the region, considering it as one paragraph.
M-o M-s
Center a line.

The command M-q (fill-paragraph) fills the current paragraph. It redistributes the line breaks within the paragraph, and deletes any excess space and tab characters occurring within the paragraph, in such a way that the lines end up fitting within a certain maximum width.

Normally, M-q acts on the paragraph where point is, but if point is between paragraphs, it acts on the paragraph after point. If the region is active, it acts instead on the text in the region. You can also call M-x fill-region to specifically fill the text in the region.

M-q and fill-region use the usual Emacs criteria for finding paragraph boundaries (see Paragraphs). For more control, you can use M-x fill-region-as-paragraph, which refills everything between point and mark as a single paragraph. This command deletes any blank lines within the region, so separate blocks of text end up combined into one block.

A numeric argument to M-q tells it to justify the text as well as filling it. This means that extra spaces are inserted to make the right margin line up exactly at the fill column. To remove the extra spaces, use M-q with no argument. (Likewise for fill-region.)

The maximum line width for filling is specified by the buffer-local variable fill-column. The default value (see Locals) is 70. The easiest way to set fill-column in the current buffer is to use the command C-x f (set-fill-column). With a numeric argument, it uses that as the new fill column. With just C-u as argument, it sets fill-column to the current horizontal position of point.

The command M-o M-s (center-line) centers the current line within the current fill column. With an argument n, it centers n lines individually and moves past them. This binding is made by Text mode and is available only in that and related modes (see Text Mode).

By default, Emacs considers a period followed by two spaces or by a newline as the end of a sentence; a period followed by just one space indicates an abbreviation, not the end of a sentence. Accordingly, the fill commands will not break a line after a period followed by just one space. If you set the variable sentence-end-double-space to nil, the fill commands will break a line after a period followed by one space, and put just one space after each period. See Sentences, for other effects and possible drawbacks of this.

If the variable colon-double-space is non-nil, the fill commands put two spaces after a colon.

To specify additional conditions where line-breaking is not allowed, customize the abnormal hook variable fill-nobreak-predicate (see Hooks). Each function in this hook is called with no arguments, with point positioned where Emacs is considering breaking a line. If a function returns a non-nil value, Emacs will not break the line there. Functions you can use there include: fill-single-word-nobreak-p (don't break after the first word of a sentence or before the last); fill-single-char-nobreak-p (don't break after a one-letter word); and fill-french-nobreak-p (don't break after ‘(’ or before ‘)’, ‘:’ or ‘?’).