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A list consisting of one or more list of exactly two elements each is called an associative list, or a-list. A-lists are a common data structure used for indexing information; the first member of a pair is called the key, while the second member is the data being indexed. For example, a list of programmers and the systems they developed may go like this:
(define OS-Developers '(("Linus Torvalds" "Linux")
("Richard Stallman" "HURD")
("Bill Jolitz" "FreeBSD")
("Pype-Clicker" "Clicker32")
("Tim Robinson" "Mobius")
("Solar" "Pro-POS")
("Jay Osako" "Thelema")
(" Robert Szeleney" "SkyOS")))
The function (assoc) takes an a-list and a key value, and returns the key-value pair (or #f if it is not found in the list).
> (assoc "Jay Osako" OS-Developers)
("Jay Osako" "Thelema")
> (assoc "HURD" OS-Developers)
#f
> (assoc "Richard Stallman" OS-Developers)
("Richard Stallman" "HURD")