The Fourth Book of Occult Philosophy

Book IV.

...consist of the same magnitude and nature: then, once the number of those stars has been placed according to their location and order, let the lines be drawn together in the likeness of the signified image, as fully as this can be done.

Truly, the Characters Character: a magical seal or signature believed to represent the essence of a celestial being which are drawn forth according to the name of a spirit Spirit: in this context, an intelligence or angelic being associated with a specific planet or star are composed by the following table, by giving to each letter of the name that which corresponds to it from the table: as is easily apparent to the viewer.

But there is a significant difficulty in this, namely when a letter of the name falls upon the line of figures or of letters, so that we may know which figure or which letter ought to be taken. This, however, shall be determined in this way: For if a letter falls into the line of letters, consider which position this letter holds in the order of the Name—for example, if it is the second or third letter—then consider how many letters the name itself comprises, such as five or seven. Multiply these numbers by each other, and triple the product. Then cast the whole sum original: proiice; this refers to a method of counting through a list (like the alphabet) repeatedly until the total number is reached from the beginning of the letters, according to the succession of the Alphabet: and whichever letter that number falls upon, this is the one which ought to be placed in the Character of that Spirit. If, however, some letter of the name [falls] in...

The text describes a form of "onomancy" or name-magic, where the letters of a spirit's name are converted into geometric shapes or different letters through a mathematical formula (Position × Total Length × 3), then counted out across the alphabet to find the "secret" letter for the sigil.