Grammar Girl here. I'm In Yon Fugherty, your friendly guide to the English language. Today, we're

going to look at why the letter X stands for the unknown in algebra, plus some X-related word

info, and then we'll look at why we say how come for why. But first, did you know that right now

you can get a free month of the Grammar Girl Patreon? And when you do, you also get a free super

list of 100 books that are the best books our podcast guests have ever read. When you're

finally relaxing and getting ready to settle in with a

good book before the new year, you'll love this list. The links are in the show notes, which is also

called the show description or the episode details on some platforms. This first segment is by

Professor of Mathematics, Peter Schumer.

Even though X is one of the least used letters in the English alphabet, it appears throughout American culture, from Stanley's X-Men superheroes to the X-Files TV series. The letter X often symbolizes

something unknown with an air of mystery that can be appealing. Just look at Elon Musk with

SpaceX, Tesla's Model X, and now X as a new name for Twitter. You might be most familiar

with X from math class. Many algebra problems use X as a variable to stand in for an unknown quantity, but why is X the letter chosen for this role? When and where did this convention begin? Well, there are a few different explanations that math enthusiasts have put forward, some citing

translation, others pointing to a more typographic origin. Each theory has some merit,

but historians of mathematics, like me, know that it's difficult to say for sure how X got

its role in modern algebra. Algebra today is a branch of math in which

abstract symbols are manipulated, using arithmetic to solve different kinds of equations. But many

ancient societies had well-developed mathematical systems and knowledge without symbolic notation.

All ancient algebra was rhetorical.

Mathematical problems and solutions were completely written out in words as part of a little story,

much like the word problems you might see in elementary school.

Ancient Egyptian mathematicians,

...