Ehyeh is the name given to Moses at the bush, 'I will be what I will be', and its 21 is the number of the name that is entirely in the future tense. The Hermetic Qabalah places it at Kether, so the smallest of the divine names sits at the top of the tree, and the tradition reads the future tense as the point of the emanation before anything has come out. Its 21 is also three sevens, the number of the doubles tripled.