  In this note we describe a general elementary procedure to attach a fusion
ring to any Kac-Moody algebra of affine type. In the case of untwisted affine
algebras, they are usual fusion rings in the literature. In the case of twisted
affine algebras, they are exactly the twisted fusion rings defined by the
author in [Ho2] via tracing out diagram automorphisms on conformal blocks for
appropriate simply-laced Lie algebras. We also relate the fusion ring to the
modular S-matrix for any Kac-Moody algebra of affine type.
