  Graphs which generalize the simple or affine Dynkin diagrams are introduced.
Each diagram defines a bilinear form on a root system and thus a reflection
group. We present some properties of these groups and of their natural "Coxeter
element". The folding of these graphs and groups is also discussed, using the
theory of C-algebras. (Proceedings of the Taniguchi Symposium {Topological
Field Theory, Primitive Forms and Related Topics}, Kyoto Dec 1996)
