With the Wunderbars range of constant tension stretcher bars you'll never have to worry about a slack canvas again. Wunderbars patented technology not only does the stretching of the canvas for you but also ensures that the canvas stays taught and sag free. Throw away your canvas stretching pliers and watch Wunderbars do all of the stretching for you. Simply pull the pin and watch as the frame expands to its optimal stretching point. Time saving and ideal for anyone who's not stretched canvas before. Wunderbars really are the Rolls Royce of stretching canvas solutions.