Let L/K be a tame and Galois extension of number fields with group G. It is well-known that any ambiguous ideal in L is locally free over O K G (of rank one), and so it defines a class in the locally free class group of O K G, where O K denotes the ring of integers of K. In this paper, we shall study the relationship among the classes arising from the ring of integers O L of L, the inverse different D −1 L/K of L/K, and the square root of the inverse different A L/K of L/K (if it exists), in the case that G is abelian. They are naturally related because A