Composition most commonly refers to a binary operation on primitive equivalence classes of forms of the same discriminant, one of the deepest discoveries of Gauss, which makes this set into a finite abelian group called the form class group (or simply class group) of discriminant formula_28. Class groups have since become one of the central ideas in algebraic number theory. From a modern perspective, the class group of a fundamental discriminant formula_28 is isomorphic to the narrow class group of the quadratic field formula_124 of discriminant formula_28. For negative formula_28, the narrow class group is the same as the ideal class group, but for positive formula_28 it may be twice as big.