The clamp comprises an inverted U-shaped base having spaced apart legs defining an entrance with a clamping element mounted on one leg of the base for displacement towards and away from a clamping surface on the other leg of the base for clamping a sheet material article thereagainst. The clamping element has an outer surface which is guided on a guide surface disposed on the base leg which is opposite the clamping surface and is retained in contact therewith by means of a tension member. The guide surface has at least one or preferably a plurality of sections extending at an angle or angles relative to the clamping surface.
