A new method for the analysis of folding of seismically mapped horizons is described. Based on a model of developable surfaces, the local geometrical properties are determined by analyzing the variation of dip and strike along linear strips on the surface. By considering strips in different directions, a plunge line (the approximation to the generatrix of a developable fold) is identified as the direction associated with the least variation of surface attitude. The map pattern obtained by analyzing the plunge and trend of plunge lines across an area allows the identification of domains where folding accords with a developable geometry. Such domains are recognized from straight plunge lines, defining convergent or parallel patterns. Deviations from these patterns correspond to regions of structural complexity associated with ductile or brittle straining of the horizon. We suggest that plunge-line analysis may offer a useful technique for automatic fault recognition.