A Shamrock Chrismon Symbol

       Trefoil (from Latin trifolium, "three-leaved plant", French trèfle, Italian trifoglio, German Dreiblatt and Dreiblattbogen, Dutch klaver, "clover", same as clubs) 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.

       Traditionally, shamrock is said to have been used by Saint Patrick to illustrate the Christian doctrine of the Holy Trinity when Christianizing Ireland in the 5th century. The first evidence of a link between St Patrick and the shamrock appears in 1675 on the St Patrick's Coppers or Halpennies. These appear to show a figure of St Patrick preaching to a crowd while holding a shamrock, presumably to explain the doctrine of the Holy Trinity. In pagan Ireland, three was a significant number and the Irish had many triple deities, which could have aided St Patrick in his evangelization efforts. Patricia Monaghan states that "There is no evidence that the clover or wood sorrel (both of which are called shamrocks) were sacred to the Celts". However, Jack Santino speculates that "The shamrock was probably associated with the earth and assumed by the druids to be symbolic of the regenerative powers of nature ... Nevertheless, the shamrock, whatever its history as a folk symbol, today has its meaning in a Christian context. Pictures of Saint Patrick depict him driving the snakes out of Ireland with a cross in one hand and a sprig of shamrocks in the other." Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the triskele when he uses the shamrock to explain the Trinity".

       The first written mention of the link does not appear until 1681, in the account of Thomas Dineley, an English traveller to Ireland. Dineley writes:

"The 17th day of March yeerly is St Patricks, an immoveable feast, when ye Irish of all stations and condicions were crosses in their hatts, some of pinns, some of green ribbon, and the vulgar superstitiously wear shamroges, 3 leav'd grass, which they likewise eat (they say) to cause a sweet breath."

       There is nothing in Dineley's account of the legend of St. Patrick using the shamrock to teach the mystery of the Holy Trinity, and this story does not appear in writing anywhere until a 1726 work by the botanist Caleb Threlkeld. Threlkeld identifies the shamrock as White Field Clover (Trifolium pratense album ) and comments rather acerbically on the custom of wearing the shamrock on St. Patrick's Day:

"This plant is worn by the people in their hats upon the 17. Day of March yearly, (which is called St. Patrick's Day.) It being a current tradition, that by this Three Leafed Grass, he emblematically set forth to them the Mystery of the Holy Trinity. However that be, when they wet their Seamar-oge, they often commit excess in liquor, which is not a right keeping of a day to the Lord; error generally leading to debauchery."

• Scriptural References for The Holy Trinity here