I have a question on the use of apostrophe to indicate the plural of a mathematical object.

Consider the sentence: "There are many values of X such that the statement is true".

In math, often we re-write this sentence as "There are many X's such that the statement is true".

Now, my question is: which one is correct (or preferred) between

"There are many X's such that the statement is true"


"There are many Xs such that the statement is true"

In case they are both wrong, please explain.


A single letter X in mathematical notation indicates an unknown. You would write the plurals as Xs or perhaps as Xs (the former has both X and s in bold; the latter has only the X in bold.)

Note how Xs and Ys are used at our sister mathematics site.

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    While sometimes adding 's can be acceptable for plurals of single letters (see, for instance answers to this question), particularly in mathematics, where it is common to use apostrophes/primes to denote other variables (e.g. X, X', X'' etc.), or for differentiation, it would be liable to cause confusion. – TripeHound Aug 1 '19 at 11:21
  • Good point! Mathematics has a complex orthography and uses some subtle typography to express this. That unknown x is different from X and has numerous variations (which I can't express well typographically with MathJax). Besides X, there's "X-prime," "X-double-prime," "X-bar," "X vector," "X-hat" (you get the picture). I think Xs and xs could lead to excess. – rajah9 Aug 2 '19 at 12:04

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