If you choose to have a series of numbered backup files, backup file
names contain `.~', the number, and another `~' after the original file
name.  Thus, the backup files of `eval.c' would be called `eval.c.~1~',
`eval.c.~2~', and so on, all the way through names like `eval.c.~259~'
and beyond.  The variable `backup-directory-alist' applies to numbered
backups just as usual.