By default, Emacs starts in multibyte mode: it stores the contents of buffers and strings using an internal encoding that represents non-ASCII characters using multi-byte sequences. Multibyte mode allows you to use all the supported languages and scripts without limitations.
Under very special circumstances, you may want to disable multibyte character support, for a specific buffer. When multibyte characters are disabled in a buffer, we call that unibyte mode. In unibyte mode, each character in the buffer has a character code ranging from 0 through 255 (0377 octal); 0 through 127 (0177 octal) represent ASCII characters, and 128 (0200 octal) through 255 (0377 octal) represent non-ASCII characters.
To edit a particular file in unibyte representation, visit it using
find-file-literally. See Visiting Functions. You can
convert a multibyte buffer to unibyte by saving it to a file, killing
the buffer, and visiting the file again with
find-file-literally. Alternatively, you can use C-x
<RET> c (
universal-coding-system-argument) and specify
‘raw-text’ as the coding system with which to visit or save a
file. See Specifying a Coding System for File Text. Unlike
a file as ‘raw-text’ doesn't disable format conversion,
uncompression, or auto mode selection.
The buffer-local variable
nil in multibyte buffers, and
nil in unibyte ones.
The mode line also indicates whether a buffer is multibyte or not.
With a graphical display, in a multibyte buffer, the portion of the
mode line that indicates the character set has a tooltip that (amongst
other things) says that the buffer is multibyte. In a unibyte buffer,
the character set indicator is absent. Thus, in a unibyte buffer
(when using a graphical display) there is normally nothing before the
indication of the visited file's end-of-line convention (colon,
backslash, etc.), unless you are using an input method.
You can turn off multibyte support in a specific buffer by invoking the
toggle-enable-multibyte-characters in that buffer.