In set builder notation, “x|x” means “x, so that x.” It is used to make lists of numbers and to define domains when making graphs.
“x|x” goes between the curly brackets that start and end a set. The first x stands for all the possible numbers in a set. The second x is followed by a qualifier that narrows the set, such as “less than” or “greater than” a certain amount. For example, x|x5 can be written as the set of all x such that x is less than 5. The variable x in “x|x” can be changed to any other letter (x|x, a|a, b|b, and c|c all mean the same thing), and the bar in the middle can be changed to a: sign (x:x is the same as “x|x”).