Keystone: the central stone of an arch.
King-Post: in timber roof-construction; a central post, resting on one of the Tie-beams (which see) to support the ridge. See Queen-Post.
[L]ady-Chapel: a chapel in an English cathedral, dedicated to the Virgin Mary, usually situated at the back of the altar.
Lancet: applied to an arch or window that has a sharply pointed, lance-shaped opening.
Lantern: a superstructure that rises above the roof level, open below and admitting light through its sides. Called in Spanish a Cimborio.
Lierne-rib: See Rib.
Lintel: the horizontal beam, supported on two uprights or posts, covering an opening and supporting weight, e.g., the top member of the frame of a doorway or window.
Loggia: a covered gallery, open to the air on one or more sides.
Longitudinal: parallel to the direction of the main axis. Specifically applied to the arches and ribs of the vaulting of a nave or aisle in the direction East or West. Compare Diagonal and Transverse.
Louver: a lantern-like cupola on the roof of a mediæval building, originally the flue for smoke from the fire in the centre of the hall.