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.
