Though one of the least-used letters, X has a remarkable way of getting attention. Last year we talked about the varied uses of X: Gen X, Xbox, XOXO, the X chromosome. British dramatist Ben Jonson wanted to remove the 24th letter from the alphabet saying, that x “is rather an abbreviation, or way of short writing, than a letter: for it hath the sound of c and s, or k and s.” With all due respect to Ben Jonson, there’s so much more to X: the fact that the letter and the mathematical symbol × are actually different entities.

Technically, the algebraic symbol × is not identical to the letter x, though they are orthographically related. The difference is subtle; the shape of the letter x is not symmetrical. If you look at it closely, you will see that the top portion above the crossed lines is slightly smaller than the lower portion. Our eyes don’t usually detect it, but this slight variation makes the letter look more stable and more readable.

The mathematical symbol × is two perfectly crossed lines with equal space above and below. When did we start using ×? Arabic mathematicians invented algebra in the 800s. The word algebra came from the Arabic word “al jebr” meaning “reunion of broken parts.” These early mathematicians used the Arabic word *šhay’* (which means “thing”) to represent the unknown. When the concepts of algebra reached Spain in the 1000s, the word was particularly hard to pronounce. It makes the “sh” sound which is unusual in Spanish, so Spanish scholars replaced that sound with the Greek chi. In Latin, the Greek chi became the letter x.

For a more detailed explanation, see Terry Moore’s recent video at TED.

Do you use the letter x to represent the unknown? What do you think of this slippery little symbol?