X marks the unknown in algebra, but X's origins are a math mystery

Even though "x" is one of the least-used letters in the English alphabet, it appears throughout American culture—from Stan Lee'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.