The three mother letters are Aleph, Mem, and Shin. They represent principle, contrary principle, and balance. They are the elements air, water, and fire. The heavens are created from fire. The earth is created from water.
Air acts as the intermediary between fire and water.
The seven double letters are Beth, Gimel, Daleth, Caph, Pe, Resh, and Tau. They have double sounds and double meanings. Beth is life and death. Gimel is peace and evil. Daleth is wisdom and folly.