Why are $X$ and $Y$ commonly used as mathematical placeholders?
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I realize that $X$ and $Y$ are relatively popular terms when wanting to use a placeholder for an unknown English or math term. What is the origin of this term, and why was it $X$ and $Y$ ; why not the other letters?
They became popular because of René Descartes’ usage in his La Géométrie. The letters at the end of the alphabet are chosen as the variables, while those at the beginning are constants. There is speculation about why this might have been done. It is likely to allow the largest number of sequential letters without overlap between the two sets.
Why x became the most common is unknown. Some sources attempt to draw a line from the Arabic word for unknown through the Greek letter chi (which resembles a capital X), but the claims are unsubstantiated (Arabic being the source of our numerals and Greek being the common letter set for variables).
