Set of 16 keys. 3D Pro Stretcher Bars K2 and K3 have double tongue and groove joints, therefore 4 keys are needed to key-out each corner, or 16 total keys per frame. This key or wedge is made of solid Pine and is 1/4" thick. It is designed to insert into the inside perimeter of the frame into the little slots of the Stretcher Bar tenon. Use these keys to increase the tension level of the canvas. By inserting the wedges and lightly hammering on the keys, the frame will enlarge and thereby increase the tension on the canvas. We recommend the use of this product only when the canvas is loose and needs to be tensioned. We also recommend tapping the keys with a rubber mallet to reduce the chance of splitting the wood.