**In math, "x|x" means "x, such that x" in set builder notation.** It is used when building lists of numbers and defining domains when graphing.

The term "x|x" is put between curly brackets that begin and end a set. The first x stands for all possible numbers in a set; the second is followed by a qualifier that narrows the set, such as "less than" or "greater than" a certain amount. For example, the set of all x such that x is less than 5 can be written as {x|x<5}. The variable x in "x|x" can be replaced with any other letter (x|x, a|a, b|b, c|c all mean the same thing) and the bar in the middle can be replaced with a : sign (x:x is the same as x|x).