Coding information

To store and process information in a compact, convenient way people use signs: letters stand for sounds, notes for music, formulas for laws. If the meaning of a sign can be guessed from its look, it is a pictogram; if the meaning is known only by agreement, it is a symbol, e.g. the letter A or the digit 5. Changing information into another form by exact rules is coding, turning coded information back is decoding, and the sign or sequence of signs assigned to a character is its code. If every character has a code of the same length, the coding is uniform; if the lengths differ, it is non-uniform. Coding made to keep information secret is encryption, and opening it is decryption. The ancient Greeks used the scytale (a strip wound round a stick; unwound, the text shows its letters in a different order), and Julius Caesar replaced each letter by the one three places later in the alphabet. Morse code (telegraph, late 1830s) writes letters with dots and dashes and is non-uniform: the letter E is a single dot, while Q takes four signs. Another simple way is to write letters by their number in the alphabet; mixing up which number goes to which letter makes the code harder to find.