The four-leaf clover is an uncommon variation of the common, three-leaved clover. According to tradition, such leaves bring good luck to their finders, especially if found accidentally. According to legend, each leaflet represents something: the first is for hope, the second is for faith, the third is for love, and the fourth is for luck. The three leaf clover, or "Shamrock", was also what Saint Patrick used to represent the Holy Trinity. Clovers can have more than four leaflets: the most ever recorded is twenty-one, a record set in June 2008 by the same man who held the prior record and the current Guinness World Record of eighteen. Unofficial claims of discovery have ranged as high as twenty-seven.
It has been estimated that there are approximately 10,000 three-leaf clovers for every four-leaf clover, however this probability has not deterred collectors who have reached records as high as 160,000 four-leaf clovers. It is debated whether the fourth leaflet is caused genetically or environmentally. Its relative rarity suggests a possible recessive gene appearing at a low frequency. Alternatively, four-leaf clovers could be caused by somatic mutation or a developmental error of environmental causes. They could also be caused by the interaction of several genes that happen to segregate in the individual plant. It is possible all four explanations could apply to individual cases.
Certain companies produce four-leaf clovers using different means. Richard Mabey alleges, in Flora Britannica, that there are farms in the US which specialize in four-leaf clovers, producing as many as 10,000 a day (to be sealed in plastic as "lucky charms") by feeding a secret, genetically-engineered ingredient to the plants to encourage the aberration (there are, however, widely-available cultivars that regularly produce leaves with multiple leaflets – see below). Mabey also states that children learn that a five-leaved clover is even luckier than a four-leaved one. Five-leaf clovers are less commonly found naturally than four-leaf clovers; however, they, too, have been successfully cultivated.
Other plants may be mistaken for, or misleadingly sold as, "four-leaf clovers"; for example, Oxalis tetraphylla is a species of wood sorrel with leaves resembling a four-leaf clover. Other species that have been sold as "four-leaf clovers" include Marsilea quadrifolia and Oxalis deppei.
There are some cultivars of white clover (Trifolium repens) which regularly produce more than three leaflets, including purple-leaved T. repens 'Purpurascens Quadrifolium' and green-leaved T. repens 'Quadrifolium'.
Trifolium repens 'Good Luck' is a cultivar which has three, four, or five green, dark-centred leaflets per leaf.