Historians believe Lewis coded his 1801 review of all officers commissioned in the United States Army for Jefferson's use in reducing the size of the military.
In response to the opening of his mail by European postmasters during his service as Minister to France (1784-1789), Jefferson began to rely heavily on codes to send important messages. His belief in the practice was strong enough to prompt him to invent his own enciphering device, the Wheel Cipher (a reproduction of which is seen at right) in the early 1790s. While it appears Jefferson rarely, if ever, used the Wheel Cipher, he continued to use codes throughout his public career, including two that are associated with his communications with Meriwether Lewis:
Jefferson sent Lewis a cipher to use for encoding messages as the Expedition moved through what then was still French territory.