Branches of number symbolism
Systems that associate numbers with letters
In many magical system, each letter is associated with a number and various operations are performed on words and phrases using those numbers. Old Greek and Hebrew don't have seperate signs for numbers, the letters are used instead. So the association between numbers and letters seemed natural to those people.
The hebrew version of this is called Gematria. Basically, the letter values of a word or phrase are summed up, and the resulting number is sometimes subjected to number reduction. Phrases or words that reduce to the same number are thought to be the same, opposites, or have a mysthical connection. Originally Gematria was used as a method to analyse and interpret the Thanak, the jewish bible. The bible is seen as the direct word of god, but heavily veiled or even kind of encrypted. This is one of the basic methods to divine it's true meanings jewish kabbalists use. Later, the same practices were (and are) used for divination, character analysis and similar purposes.
The greek version is called Isopsephia.
Usually, such systems are not seen as a part of number symbolism. But we must remember that for the inventors of those systems, letters and numbers were basically the same thing. So it seems legitimate to call this a branch of number symbolism.
Numbers in European folklore and medieval magick
Number symbolism outside the Western mystery tradition
Number symbolism in Tantra
Numbers in modern magic
Several systems of gematria have been developed that are meant to be used with the english language.