  We review a reduction formula by Petersson that reduces the calculation of a
one-loop amplitude with N external lines in n<N space-time dimensions to the
case n=N and give it a geometric interpretation. In the case n=N the
calculation of the euclidean amplitude is shown to be equivalent to the
calculation of the volume of a tetrahedron spanned by the momenta in
(n-1)-dimensional hyperbolic space. The underlying geometry is intimately
linked to the geometry of the reduction formula.
