James Baquet

    Becky is chatting with her classmate Lily in the common room of their dorm.

    Becky: Hi, Lily. Are you enjoying the Christmas holiday season?

    Lily: I sure am. But Becky, I was wondering: Why do some people call it “Xmas” with an “X?”

    Becky: The letters “ch” in “Christmas” — as well as in psychology, chronology, and other words that come from Greek — represent the letter “chi,” which is written like our “X.”

    Lily: I see. So the “X” is like an initial for “Christ?”

    Becky: That’s right. You know, the letter X has a ton of interesting uses in English.

    Lily: Like the Xbox?

    Becky: That’s one. It was developed by a group of Microsoft engineers called the “DirectX Group.” In fact, it was originally called the “DirectX Box.”

    Lily: I had no idea! Also, isn’t “X” used to indicate the female chromosome?

    Becky: That’s right! Women are “XX,” and men are “XY.”

    Lily: So the father determines the sex of the child by donating either an X or a Y?

    Becky: Yes! Here’s another one: “X” is often used to indicate the unknown.

    Lily: Why is that?

    Becky: No one’s sure. One theory is that it goes back to Descartes, the philosopher and mathematician. He used X, Y, and Z to indicate the axes in three dimensions.

    Lily: Like on a graph.

    Becky: Uh-huh. He may have been thinking about how we use “A, B, C” so often, and just skipped to the other end of the alphabet. Since “X” is first, it’s most commonly used in, for example, algebra.

    Lily: Like “solving for X?”

    Becky: Right. For example, in “X - 3 = 7” (x minus three equals seven), “X” equals 10.

    Lily: Got it. It’s also used to indicate multiplication, right?

    Becky: Right. “2 x 2 = 4” (two times two equals four).

    Lily: What are some other uses of “X?”

    Becky: We have an old tradition in literature: If someone draws a treasure map, the location of the treasure will be marked with an “X.”

    Lily: Oh, “X marks the spot!”

    Becky: Exactly.

    Lily: Even now, when people draw me a map to find a coffee shop or a bookstore, they often use an “X” for the starting and ending points.

    Becky: Also, in the old days, if a person wasn’t able to write, he could sign an “X” on a legal document.

    Lily: I guess because it’s easy to write?

    Becky: Maybe.

    Lily: Sometimes when people write me a note, they end it “XOXOXO.” What does that mean?

    Becky: Well, the “Os” are supposed to be hugs.

    Lily: I see. Like someone’s arms make a circle.

    Becky: Yes. And the “Xs” are kisses!

    Lily: Like drawing the lips together?

    Becky: Could be.