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[?].

Larger Birdsfoot trefoil