If you choose to have a single backup file (this is the default), the backup file's name is constructed by appending `~' to the file name being edited; thus, the backup file for `eval.c' would be `eval.c~'.
If you choose to have a series of numbered backup files, backup file names are made by appending `.~', the number, and another `~' to the original file name. Thus, the backup files of `eval.c' would be called `eval.c.~1~', `eval.c.~2~', and so on, through names like `eval.c.~259~' and beyond.
If protection stops you from writing backup files under the usual names, the backup file is written as `%backup%~' in your home directory. Only one such file can exist, so only the most recently made such backup is available.
The choice of single backup or numbered backups is controlled by the
version-control. Its possible values are
You can set
version-control locally in an individual buffer to
control the making of backups for that buffer's file. For example,
Rmail mode locally sets
never to make sure
that there is only one backup for an Rmail file. See section Local Variables.
If you set the environment variable
VERSION_CONTROL, to tell
various GNU utilities what to do with backup files, Emacs also obeys the
environment variable by setting the Lisp variable
accordingly at startup. If the environment variable's value is `t'
or `numbered', then
t; if the
value is `nil' or `existing', then
nil; if it is `never' or `simple', then
For files under version control (see section Version Control), the
vc-make-backup-files determines whether to make backup
files. By default, it is
nil, since backup files are redundant
when you store all the previous versions in a version control system.
See section Editing with Version Control.