We give, in Sections 2 and 3, an english translation of: Classes généralisées invariantes, J. Math. Soc. Japan, 46, 3 (1994), with some improvements and with notations and definitions in accordance with our book: Class Field Theory: from theory to practice, SMM, Springer-Verlag, 2 nd corrected printing 2005. We recall, in Section 4, some structure theorems for finite Zp[G]-modules (G ≃ Z/p Z) obtained in: Sur les ℓ-classes d'idéaux dans les extensions cycliques relatives de degré premier ℓ, Annales de l 'Institut Fourier, 23, 3 (1973). Then we recall the algorithm of local normic computations which allows to obtain the order and (potentially) the structure of a p-class group in a cyclic extension of degree p. In Section 5, we apply this to the study of the structure of relative p-class groups of Abelian extensions of prime to p degree, using the Thaine-Ribet-Mazur-Wiles-Kolyvagin "principal theorem", and the notion of "admissible sets of prime numbers" in a cyclic extension of degree p, from: Sur la structure des groupes de classes relatives, Annales de l'Institut Fourier, 43, 1 (1993). In conclusion, we suggest the study, in the same spirit, of some deep invariants attached to the p-ramification theory (as dual form of non-ramification theory) and which have become standard in a p-adic framework. Since some of these techniques have often been rediscovered, we give a substantial (but certainly incomplete) bibliography which may be used to have a broad view on the subject.We shall see the field L given, via Class Field Theory, by some Artin group of K (e.g., the Hilbert class field H + K of K associated with the group of principal ideals, in the narrow sense, any ray class field H + K,m associated with a ray group modulo a modulus m of k, in the narrow sense, or more generally any subfield L of these canonical fields, defining Gal(H + K,m /L) by means of a sub-G-module H of the generalized class group Cℓ + K,m ≃ Gal(H + K,m /K)). We intend to give, from the arithmetic of k and elementary local normic computations in K/k, an explicit formula for #Gal(L/K) G = #(Cℓ + K,m /H) G .This order is the degree, over K, of the maximal subfield of L (denoted L ab ) which is Abelian over k.