Two models to explain the progressive deformation of syntectonic quartz veins are derived from conventional theories for simple and pure shears. The simple-shear model is based on reorientation and changes in length of linear vein elements, and predicts initial orientations of veins for imposed shear strains, elongations and strain ratios. The pure-shear model considers changes in length of lines variably orientated relative to the maximum compression direction, and yields estimates of elongation strains and strain ratios. Expectations of both models are different, as illustrated by analysis of quartz veins from the Rhoscolyn Anticline, Anglesey, NW Wales. The simple-shear model recognizes three distinct initial orientations, which predict different strains across the fold; the pure-shear model suggests veins were initially sub-parallel to the principal compression direction and predicts effectively constant strains across the fold. In addition, both models predict different patterns of fold vergence: for simple shear, vergence depends on magnitude and direction of shearing and may exhibit complex patterns; for pure shear, vergence patterns are predicted to be essentially constant. In general, the predictions of either model are critically dependent on the origin of the veins, particularly relative to the formation of the Rhoscolyn Anticline.