Q.6 Unibyte and Multibyte Non-ASCII characters

When multibyte characters are enabled, character codes 0240 (octal) through 0377 (octal) are not really legitimate in the buffer. The valid non-ASCII printing characters have codes that start from 0400.

If you type a self-inserting character in the range 0240 through 0377, or if you use C-q to insert one, Emacs assumes you intended to use one of the ISO Latin-n character sets, and converts it to the Emacs code representing that Latin-n character. You select which ISO Latin character set to use through your choice of language environment (see section Q.3 Language Environments). If you do not specify a choice, the default is Latin-1.

If you insert a character in the range 0200 through 0237, which forms the eight-bit-control character set, it is inserted literally. You should normally avoid doing this since buffers containing such characters have to be written out in either the emacs-mule or raw-text coding system, which is usually not what you want.