38.16 Usual Display Conventions

The usual display conventions define how to display each character code. You can override these conventions by setting up a display table (see section 38.17 Display Tables). Here are the usual display conventions:

• Character codes 32 through 126 map to glyph codes 32 through 126. Normally this means they display as themselves.

• Character code 9 is a horizontal tab. It displays as whitespace up to a position determined by tab-width.

• Character code 10 is a newline.

• All other codes in the range 0 through 31, and code 127, display in one of two ways according to the value of ctl-arrow. If it is non-nil, these codes map to sequences of two glyphs, where the first glyph is the ASCII code for `^'. (A display table can specify a glyph to use instead of `^'.) Otherwise, these codes map just like the codes in the range 128 to 255.

  On MS-DOS terminals, Emacs arranges by default for the character code 127 to be mapped to the glyph code 127, which normally displays as an empty polygon. This glyph is used to display non-ASCII characters that the MS-DOS terminal doesn't support. See section `MS-DOS and MULE' in The GNU Emacs Manual.

• Character codes 128 through 255 map to sequences of four glyphs, where the first glyph is the ASCII code for `\', and the others are digit characters representing the character code in octal. (A display table can specify a glyph to use instead of `\'.)

• Multibyte character codes above 256 are displayed as themselves, or as a question mark or empty box if the terminal cannot display that character.

The usual display conventions apply even when there is a display table, for any character whose entry in the active display table is nil. Thus, when you set up a display table, you need only specify the characters for which you want special behavior.

These display rules apply to carriage return (character code 13), when it appears in the buffer. But that character may not appear in the buffer where you expect it, if it was eliminated as part of end-of-line conversion (see section 33.10.1 Basic Concepts of Coding Systems).

These variables affect the way certain characters are displayed on the screen. Since they change the number of columns the characters occupy, they also affect the indentation functions. These variables also affect how the mode line is displayed; if you want to force redisplay of the mode line using the new values, call the function force-mode-line-update (see section 23.3 Mode Line Format).

User Option: ctl-arrow

This buffer-local variable controls how control characters are displayed. If it is non-nil, they are displayed as a caret followed by the character: `^A'. If it is nil, they are displayed as a backslash followed by three octal digits: `\001'.

Variable: default-ctl-arrow

The value of this variable is the default value for ctl-arrow in buffers that do not override it. See section 11.10.3 The Default Value of a Buffer-Local Variable.

User Option: indicate-empty-lines

When this is non-nil, Emacs displays a special glyph in each empty line at the end of the buffer, on terminals that support it (window systems).

User Option: tab-width

The value of this variable is the spacing between tab stops used for displaying tab characters in Emacs buffers. The value is in units of columns, and the default is 8. Note that this feature is completely independent of the user-settable tab stops used by the command tab-to-tab-stop. See section 32.17.5 Adjustable "Tab Stops".