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

• 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.
• Many algebra problems use x as a variable, to stand in for an unknown quantity.
• There are a few different explanations that math enthusiasts have put forward – some citing translation, others pointing to a more typographic origin.

Ancient unknowns

• But many ancient societies had well-developed mathematical systems and knowledge with no symbolic notation.
• All ancient algebra was rhetorical.
• Ancient Egyptian mathematicians, who are perhaps best known for their geometric advances, were skilled in solving simple algebraic problems.
• For example, problem 24 asks for the value of aha if aha plus one-seventh of aha equals 19. “Aha” means something like “mass” or “heap.” The ancient Babylonians of Mesopotamia used many different words for unknowns in their algebraic system – typically words meaning length, width, area or volume, even if the problem itself was not geometric in nature.
• One ancient problem involved two unknowns termed the “first silver thing” and the “second silver thing.” Mathematical know-how developed somewhat independently in many lands and in many languages.

So what about x?

• When Spanish scholars translated the Arabic mathematical treatises, they lacked a letter for the “sh” sound and instead chose the “k” sound.
• However, there is some evidence that casts doubt upon the theory that using x as an unknown is an artifact of Spanish translation.
• I think that the most plausible explanation is to credit the influential French scholar René Descartes for the modern use of x.
• Whatever his reasons for choosing x, Descartes greatly influenced the development of mathematics, and his mathematical writings were widely circulated.

Xtending beyond algebra

• Even if the origins of x in algebra are uncertain, there are some instances in which historians do know why x is used.
• The X in Xmas as an abbreviation for Christmas definitely does come from the Greek letter χ.