Experiments using geometrically similar models were performed in an attempt to show that ptygmatic structures can be produced by the application of external compressional stress to an originally planar vein. The experiments consisted of the deformation of a single vein in various orientations, the deformation of two veins simultaneously and the deformation of conjugate shear veins. The model experiments illustrated that the intensity of deformation of the planar veins decreased as the angle to the major compressional stress increased. The fold wave lengths and amplitudes were found to be independent of both the amount of compressive load and the strain rate. The term “ptygmatic” should be restricted to those disharmonically folded veins which show no relation to the major regional structures.