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Wikipedia:
Trefoil (from Latin trifolium, "three-leaved plant", French trèfle, Italian trifoglio, German Dreiblatt and Dreiblattbogen) is a graphic form composed of the outline of three overlapping rings used in architecture and Christian symbolism. The term is also applied to other symbols of three-fold shape.
Architecture
Trefoil 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 fourfold version of an architectural trefoil is a quatrefoil.
A trefoil combined with an equilateral triangle was also a moderately common symbol of the Christian Trinity during the late Middle Ages in some parts of Europe. Two forms of this are shown below:
Outlined
Interlaced
A dove, symbolic of the Holy Spirit, is sometimes depicted within the outlined form of the trefoil combined with a triangle.
Heraldry
The heraldic trefoil is a stylized clover. It should not be confused with the figure named in French heraldry tiercefeuille, which is a stylized flower with three petals. It differs from the heraldic trefoil in being not slipped. It could be translated as threefoil.
Trefoil in heraldry
Or a fess sable between three trefoils vert
Gules a cross flory argent between four tiercefeuilles Or
Croatian trefoil, used by the Axis-sympathetic Croatian Air Force Legion in World War II, formed from a defaced Balkenkreuz
Symbols
Symmetrical Trefoils are particularly popular as warning symbols. If a box containing hazardous material is moved around and shifted into different positions, it is still easy to recognize the symbol . Easily stenciled symbols are also favored.
Radioactive hazard trefoil
Fallout shelter trefoil
Biological hazard trefoil
Universal recycling symbol
One particular stylized form of the heraldic trefoil is used as the main element in the logo of most Girl Guiding and Girl Scouting organizations. For Girl Scouts, the three trefoil leaves represent the three-fold promise: "To serve God and my country, to help people at all times, and to live by the Girl Scout law."
While the green trefoil is considered by many to be the symbol of Ireland, the harp has much greater officially recognized status. Therefore shamrocks generally do not appear on Irish coins or postage stamps.
A trefoil is also part of the logo for Adidas Originals, which also includes the iconic three stripes.