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 (formed from three partially-overlapping circles). One of the earliest examples is in the plate tracery at Winchester (1222 - 1235). The four-fold version of an architectural trefoil is a quatrefoil.