The Trefoil reference article from the English Wikipedia on 24-Apr-2004
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Trefoil

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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 (12221235); see Quatrefoil.

Trefoil is also a common name for some kinds of clover, who have three-foiled leaves.