In non-technical usage, a 'cipher' often means the same thing as a 'code'; but in cryptography, ciphers are distinguished from codes. One 20th century source gives this explanation: a cipher is "a method in which the basic unit of concealment is the letter. In comparison, a code is a form of concealment in which the basic unit is the word". Late in the century, "codes" in this cryptographic sense became rare. 21st century cryptography mostly encrypts bitstreams.