1144. This week, we look at the origin of the letter X as the variable for the unknown in algebra. Then, we look at the phrase "how come," explaining why it's more informal than "why" and how its grammar subtly differs from other question words.

That X segment was written by Peter Schumer, Professor of Mathematics and Natural Philosophy at Middlebury College, and it originally appeared on The Conversation and appears here through a Creative Commons license.

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The origin of X in algebra. Why we say ‘how come’ for ‘why.’ Water handles.

Grammar Girl: For Writers and Language Lovers.

The origin of X in algebra. Why we say ‘how come’ for ‘why.’ Water handles.