The results of extensive investigations of the low-n ideal magnetohydrodynamic stability of the TCV, NET and DIII-D tokamaks are compared in order to identify the general qualitative effects of cross-sectional shaping on the low-n stability of elongated tokamaks. The results consistently indicate a breakdown of the simple stability picture for circular and moderately elongated cross-sections when the cross-section deviates strongly from circular. Simplified extrapolations of the standard stability picture to high elongation and triangularity and to low aspect ratio are unreliable; at high elongation, the scaling of both the current limit and the beta limit is strongly dependent on the cross-section shape. Several implications for next generation tokamaks can be drawn from the conclusions, and these are discussed briefly.

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