A subset of the real numbers is said to be a set provided is the countable intersection of open sets. The name comes from German: The G stands for Gebiet, meaning "area," while the stands for Durchschnitt, meaning "intersection."
This definition is dual to that of an Fsigma set in the sense that is a set if and only if its complement is an .
Note that both and sets are fundamental to measure theory. In particular, both types of sets are Borel sets and are considered to be the second level of the so-called Borel hierarchy.