British and American spies used secret codes and ciphers to disguise their communications.  A cipher is when letters, symbols, or numbers are used in the place of real words.  In order to decode a cipher, the recipient of the letter must have a key to know what the coded letters, symbols, or words really mean. 

In the letters to the left, Benedict Arnold used a cipher to deliver his messages secretly to John André.  The cipher's key was a standard published book, either Blackstone's Commentaries on the Laws of England or Nathan Bailey's Dictionary.  When Arnold composed his letters, he first found the word he wanted to write in the key.  Instead of writing the word directly in the letter, he wrote down the page number, the line number, and the number of the word counting over from the left.  Therefore, each secret word was represented by a series of three numbers.  For example, the second word in the letter of July 12, 1780, is "293.9.7" which stands for "wrote."  André explained these methods in a letter he wrote on May 10, 1779. 

American spies also used this method. After some letters were captured in 1779 by the British,  Benjamin Tallmadge created a code using Entick's Dictionary for the Culper Gang.

Arnold and André also used other sneaky ways to hide the real content of their letters.  Arnold and André pretended to be merchants.  Arnold deliberately did not disguise some words with the cipher so that the letters seemed to be about normal business transactions.   Anyone who intercepted these letters would see such business language and think the letters were part of routine commercial deals. 

Other British and American spies used different types of ciphers and codes to communicate secretly. Some spies made up their own pocket dictionary to encode their messages.  Each word had a corresponding number.  Others spies assigned each letter in the alphabet a corresponding number.  Some spies even transposed letters in the alphabet.  Finally, other spies changed the names of major places, so that if the letters were captured, the other side would not know the places to which the letters really referred.