For much of his long and largely secret career, Colonel William F. Friedman kept a very special photograph under the glass plate that covered his desk. At first glance, the photo looks like a standard-issue keepsake of the kind owned by anyone who has served in the military. But as a note on the back of the larger print explains, the image is a cryptogram in which people stand in for letters; and thanks to Friedman’s careful positioning, they spell out the words “KNOWLEDGE IS POWER.” (Or rather they almost do: for one thing, they were four people short of the number needed to complete the “R.”) Frieman's system allowed to designate the different forms of anything that can be divided into two classes, sorts or types.

Research_Friedman (steganography), a set on Flickr.