A structure theorem for weak buildings of spherical type

Following earlier work of Tits [8], this paper deals with the structure of buildings which are not necessarily thick; that is, possessing panels (faces of codimension 1) which are contained in two chambers, only. To every building , there is canonically associated a thick building whose Weyl group W() can be considered as a reflection subgroup of the Weyl group W() of . One can reconstruct from together with the embedding W() W(). Conversely, if is any thick building and W any reflection group containing W() as a reflection subgroup, there exists a weak building with Weyl group W and associated thick building .