Used to tighten the stretched canvas. Insert into the corner slots at the back of the canvas. Place a piece of card between the stretcher & the canvas in each corner to protect the canvas from accidental contact damage during fitting.
 
Stand the canvas upright. Using a small hammer, knock the wedges upwards into the slots while supporting the canvas with your free hand. Use the wedges to move one stretcher at a time so as to keep it square. Keep rotating the canvas while knocking in the wedges until your desired tension is reached.