Mishnah * I.8.2. [The twenty-two fundamental consonants are organized into three sets – ]
Three mothers, and seven doublets, and twelve simple ones.
Mishnah I.8.2. [I.2] Some call these sets of letters "three books of letters" - Three mothers, seven doublets, and twelve simple ones. As books, they cover all three facets of the universe's formation – underlying laws which describe potential possibilities and unite Creation, then the actual ongoing development of the Universe, and finally the Universe as we experience it and engage with it.