Let K∕F be a finite normal
extension of algebraic number fields and let CK be the ideal class group of K. There
are two fundamentally different ways to define the principal genus of CK with respect
to F. Classically the principal genus is described by norm residue symbols. By the
modern definition it is the class group of the maximal unramified extension of K
which is the composite of K with an abelian extension of F. It is shown here that the
two definitions are equivalent.