Even carefully primed canvases can sag - whether due to a change in temperature or humidity or simply the passing of time. Maintain the tension of your canvases with these classic stretcher bar keys. Made from solid wood, these 1/8" thick solid wood keys are designed for use with Masterpiece Vincent Pro and Classic Stretcher Bars. Simply insert two keys into the spaces on the inside of each frame corner and firmly tap them in using a hammer to expand the frame and increase canvas tension. Made from solid kiln-dried wood Easy to use Insert after you have primed your canvas