Play fair cipher.
A play fair cipher is a list of three numbers. The letters refer to a document that can be anything such as a newspaper to an earlier document. The first letter tells which article the word belongs in. The second number tells you what line it is in. The last letter describes the number word in the line.
The Culper Code Book
The Culper code book is a book with over 1000 terms. Each number would correspond to a word on the list. With over one thousand different combinations this was a difficult code to decode.