Our aim is to describe the image and fibres of the base change lift constructed by Langlands, for solvable extensions of number fields. A particular instance of the principle of functoriality enunciated by Lang- lands, is the base change map relating automorphic forms on GL2(Ak) to forms on GL2(AK), for an extension K/k of number fields. Using the trace formula and following earlier workby Saito and Shintani, Langlands in (L), established the existence of the base change lift BCK/k sending cuspidal automorphic rep- resentations of GL2(Ak) to automorphic representations of GL2(AK), provided K/k is a cyclic extension of prime degree. Langlands further characterized the image and the fibres of the base change map BCK/k. Given an automorphism σ of K, and an automorphic representation Π of GL2(AK), let Π σ denote the automorphic representation of GL2(AK) defined by Π σ (g )=Π (σ −1 (g)). The descent property for invariant, automorphic representations established by Langlands is that if Π is a cuspidal automorphic representation of GL2(AK), which is invariant with respect to the action of the Galois group G(K/k )o f a cyclic extension K/k of prime degree, then Π lies in the image of the base change map BCK/k from automorphic representations on GL2(Ak) to automor- phic representations on GL2(AK). Further if π1 and π2 are distinct, cuspidal automorphic representations of GLn(Ak) which base change to Π, then