# Conull set

In measure theory, a **conull set** is a set whose complement is null, i.e., the measure of the complement is zero.^{[1]} For example, the set of irrational numbers is a conull subset of the real line with Lebesgue measure.^{[2]}

A property that is true of the elements of a conull set is said to be true almost everywhere.^{[3]}