"A man is insane who writes a secret unless he conceals it from the crowd and leaves it so that it can be understood only by the studious and wise."
So wrote Roger Bacon around 1250, and he went on to list seven techniques for concealing a message from prying eyes.
Bacon's first two methods involved disguising the meaning of a message through the use of "characters and symbols" or "mysterious and symbolic words", in effect using such complex language as to make the text unintelligible to all but the initiated. He used the example of writings on alchemy, but the best modern analogy might be the use of technical or scientific jargon: imagine trying to read an advanced guide to particle physics or computer programming with no previous knowledge of the subject.
Within a few sentences the inexperienced reader would be totally lost. By employing such language, Bacon was effectively using a form of code.
The third technique suggests using only consonants in writing a message, thgh jst hw sccssfl ths frm f wrtng cld b fr sndng scrt mssgs s nt ntrly clr. The fourth technique relies on the "commingling" of letters in a transposition cipher, thereby creating an anagram of the original message.
The fifth and sixth techniques detail simple substitution ciphers, one using letters, the other employing specially devised "geometric figures", and the seventh describes the use of shorthand, which is again a type of code.