Various characters usually appear in pairs. When, for example, you insert an open parenthesis, no matter whether you are programming or writing prose, you will surely enter a closing one later. By entering both at the same time and leaving the cursor in between, Emacs can guarantee you that such parentheses are always balanced. And if you have a non-qwerty keyboard, where typing some of the stranger programming language symbols makes you bend your fingers backwards, this can be quite relieving too.
This is done by binding the first key (see Rebinding in The GNU Emacs Manual) of
the pair to skeleton-pair-insert-maybe instead of
self-insert-command. The “maybe” comes from the fact that
this at-first surprising behavior is initially turned off. To enable
it, you must set skeleton-pair to some non-nil value.
And even then, a positive argument (see Arguments in The GNU Emacs Manual) will
make this key behave like a self-inserting key
(see Inserting Text in The GNU Emacs Manual).
While this breaks with the stated intention of always balancing pairs, it
turns out that one often doesn’t want pairing to occur, when the following
character is part of a word. If you want pairing to occur even then, set
skeleton-pair-on-word to some non-nil value.
Pairing is possible for all visible characters. By default the
parenthesis ‘(’, the square bracket ‘[’, the brace
‘{’ and the pointed bracket ‘<’ all
pair with the symmetrical character, and the grave accent ‘`’
pairs with the apostrophe ‘'’. All other characters pair
themselves. This behavior can be modified by the variable
skeleton-pair-alist. This is in fact an alist of skeletons
(see Skeleton Language), with the first part of each sublist
matching the typed character. This is the position of the interactor,
but since pairs don’t need the str element, this is ignored.
Some modes have bound the command skeleton-pair-insert-maybe
to relevant keys. These modes also configure the pairs as
appropriate. For example, when typing TeX input, you’d expect the
grave accent (‘`’) to pair with the apostrophe (‘'’), while in Shell
script mode it must pair to itself. They can also inhibit pairing in
certain contexts. For example an escaped character stands for itself.