complex of groups. These are modeled on orbifolds arising from cocompact properly discontinuous actions of discrete groups on simplicial complexes that have the structure of CAT(0) spaces. The quotient of the simplicial complex has finite stabilizer groups attached to vertices, edges and triangles together with monomorphisms for every inclusion of simplices. A complex of groups is said to be developable if it arises as the quotient of a CAT(0) simplicial complex. Developability is a non-positive curvature condition on the complex of groups: it can be verified locally by checking that all circuits occurring in the links of vertices have length at least six. Such complexes of groups originally arose in the theory of Bruhat–Tits buildings; their general definition and continued study have been inspired by the ideas of Gromov. See also Bass–Serre theory Right-angled Artin group References . . . . . Translated by John Stillwell from "arbres, amalgames, SL2", written with the collaboration of Hyman Bass, 3rd edition, astérisque 46 (1983). See Chapter I.5. Geometric group theory