Syllabical and Steganographical Table
Syllabical and Steganographical Table (French: Tableau syllabique et stéganographique) is an eighteenth-century cryptographical work by P. R. Wouves. Published by Benjamin Franklin Bache in 1797, it provided a method for representing pairs of letters by numbers.
Source: Wikipedia — Syllabical and Steganographical Table (CC BY-SA 4.0)