A subset of the real numbers is said to be an set provided is the countable union of closed sets. The name comes from French: The F stands for fermé, meaning "closed," while the stands for somme, meaning "sum."
This definition is dual to that of a Gdelta set in the sense that is an set if and only if its complement is a .
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.