Trefoil
Trefoil (from Latin trifolium, three-leaved plant, French trèfle,
German Dreiblatt and Dreiblattbogen) is a term in Gothic architecture
given to the ornamental foliation or cusping introduced in the heads of window-lights,
tracery, panellings, etc., in which the center takes the form of a three-lobed leaf,
one of the earliest examples being in the plate tracery at Winchester (1222—1235);
see Quatrefoil[?].