A keystone.

That part of the plastering which is forced through between
the laths and holds the rest in place.

A position or condition which affords entrance, control, pr
possession, etc.; as, the key of a line of defense; the key of a
country; the key of a political situation. Hence, that which serves to
unlock, open, discover, or solve something unknown or difficult; as,
the key to a riddle; the key to a problem.

The last board of a floor when laid down.

That part of a mechanism which serves to lock up, make fast,
or adjust to position.

That part of an instrument or machine which serves as the
means of operating it; as, a telegraph key; the keys of a pianoforte,
or of a typewriter.

An instrument by means of which the bolt of a lock is shot or
drawn; usually, a removable metal instrument fitted to the mechanism of
a particular lock and operated by turning in its place.

i. anahtar. Have you seen my car keys? (Arabamın anahtarlarını gördün mü?)

An instrument which is turned like a key in fastening or
adjusting any mechanism; as, a watch key; a bed key, etc.

A piece of wood used as a wedge.