B ecause of its dominant role in elastic stability, the unstable-symmetric bifurcation, or cusp catastrophe, is worthy of special study. We present here a systematic perturbation analysis of the cusped imperfection-sensitivity in a general bifurcational context, to examine its movement and deformation under the influence of further imperfection parameters. The elimination of passive coordinates allows explicit expressions to be derived for the horizontal and vertical shifts and the tilting of the primary two-thirds power-law cusp. The results throw new light on the earlier experiments of Roorda, which are re-examined in some detail. Contours of constant failure load in a two-dimensional imperfection space finally allow the worst types of compound imperfection to be identified.