Twenty-one is three sevens, and it is triangular, the sum of the numbers one through six. In the tarot it is the number of the World, the final and most fortunate of the twenty-two trumps of the major arcana, depicting the dancing figure within a wreath, completion and fulfilment, the journey of the Fool arrived at its end. The number's association with attainment carries into ordinary life, where twenty-one has long marked the age of majority in the English-speaking world, the traditional coming of age, and gives its name to the card game whose object is to reach it. As three times the complete seven, twenty-one reads naturally as a heightened or perfected completeness, and its calendar and ritual uses tend to follow that sense.
Twenty-one has a Kabbalistic gematria distinct from its tarot and coming-of-age associations: the divine name Ehyeh, I Am, that God gives Moses at the burning bush in Exodus 3:14, is spelled aleph, heh, yod, heh, whose values of one, five, ten and five total twenty-one, so the number carries the weight of God's own self-declared name alongside its already-noted status as three sevens and the World of the tarot; Cheiro's 1926 Book of Numbers, working independently of the Hebrew tradition, titles the same compound number the World in his own scheme, a genuine convergence between the two systems rather than a borrowing.