While straight razors flat blades are whetted on a suspended strop (velvet knives), 1/2 or 1/1 concave blades require a hanging strop made of fine cowhide leather or extremely supple Russia leather either with a turning device for hanging up or with hemp hose on the back, which serves to align the "fin" in the direction pointing away from the straight razors. If required, the leather side can be rubbed with an extremely thin layer of fine abrasive paste (red paste) and - for a final polishing on a separate strop - with polishing paste (black paste), which is worked in with the ball of the thumb. Stropping is performed at a flat angle with the back of the straight razor laid on the strop; the straight razor is drawn in the direction away from the body. The straight razor is then turned over on its back and drawn in towards the body. Changing directions without turning the straight razor over makes the blade become round (crowned) so that the cutting properties are lost. In this case, only resharpening can help.