1911 Encyclopædia Britannica/Trefoil

TREFOIL (Lat. trifolium, three-leaved plant, Fr. trèfle, Ger. Dreiblatt and Dreiblattbogen), the term in Gothic architecture given to the ornamental foliation or cusping introduced in the heads of window-lights, tracery, panellings, &c., in which the centre takes the form of a three-lobed leaf, one of the earliest examples being in the plate tracery at Winchester (1222–1235); see Quatrefoil.