Often when setting variables (particularly Alists) or when writing macros, you need to quote most, but not all of a list.

When this is the case, quote is too “greedy”, it quotes everything with no option of unquoting bits. You can use list instead of quote, and then individually quote the bits of the list that you don’t want evaluated. So you might write:

    (list 'foo bar 'baz)

To produce a list where foo and baz are returned literally, but bar is evaluated.

This is where the backquote “`” comes in. It quotes all of a form, except for those bits which you decide you want evaluated. You can introduce evaluation with a comma “,”. So, instead of writing the above form, you might write:

    `(foo ,bar baz)

Which produces the same effect. In fact, the backquote is a reader-macro, i.e. it introduces the beginning of a form which gets macroexpanded (into the list-like syntax), we can see this with:

    (macroexpand '`("%b - " ,(getenv "USERNAME") "@" ,(getenv "USERDOMAIN")))
        => (list "%b - " (getenv "USERNAME") "@" (getenv "USERDOMAIN"))

(This from FrameTitle).

Backquotes are also useful when writing macros. Since, when using them, you can write your macro such that it looks very similar to a defun. Compare:

    (defmacro my-macro-1 (arg1 arg2)
      (list 'setq arg1 (list 'cons arg2 arg1)))


    (defmacro my-macro-2 (arg1 arg2)
      `(setq ,arg1 ,`(cons ,arg2 ,arg1)))

Comparing the macroexpansions:

    (macroexpand '(my-macro-1 list 'foo))
        => (setq list (cons 'foo list))
    (macroexpand '(my-macro-2 list 'foo))
        => (setq list (cons 'foo list))

Another backquote-specific operator is the comma-at “,@”, which acts as a “splicing” operator. This is best demonstrated with an example:

    (let ((list '(a b c d)))
      `(elt1 ,list elt2))
        => (elt1 (a b c d) elt2)

Compared to:

    (let ((list '(a b c d)))
      `(elt1 ,@list elt2))
        => (elt1 a b c d elt2)

This is particularly useful when writing macros with an &rest specifier.

Corrections, additions, comments, etc… welcome – LawrenceMitchell

Very informative section, thanks! – GlauberPrado

Short but sweet - the prefect intro to backquotes! Thanks! – halloleo