The geometry of resonance tongues is considered in, mainly reversible, vers
ions of Hill's equation, close to the classical Mathieu case. Hill's map as
signs to each value of the multiparameter the corresponding Poincare matrix
. Dy an averaging method, the geometry of Hill's map locally can be underst
ood in terms of cuspoid Whitney singularities. This adds robustness to the
result. The algorithmic nature of the averaging method enables a pull-back
to the resonance tongues of the original system. (C) 2000 Academic Press.