  We present a simple method for determining the shape of fundamental domains
of generalized modular groups related to Weyl groups of hyperbolic Kac-Moody
algebras. These domains are given as subsets of certain generalized upper half
planes, on which the Weyl groups act via generalized modular transformations.
Our construction only requires the Cartan matrix of the underlying
finite-dimensional Lie algebra and the associated Coxeter labels as input
information. We present a simple formula for determining the volume of these
fundamental domains. This allows us to re-produce in a simple manner the known
values for these volumes previously obtained by other methods.
