[en] The effect of three-dimensional geometry on the stability of ideal MHD ballooning modes is addressed. By using a class of 'local 3-D equilibria,' the effects of plasma shaping, profile variations and symmetry on local stability properties can be studied. A class of local helical axis equilibria are constructed which model the properties of a quasi-helically symmetric stellarator. Changes in the 3-D shaping allow for manipulation of the harmonic content of the local shear within the magnetic surface of interest. The presence of symmetry breaking components in the local shear are shown to have a dramatic impact on the ballooning stability boundaries. In particular, these symmetry breaking components are shown to lower first ballooning stability thresholds and potentially eliminate the second stability regime. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering