Canvas keys are also known as canvas wedges, painters keys, corner keys or canvas inserts. They are used to tighten saggy canvases. All high quality stretched canvas comes with these attached to the back, such as Fredrix Canvases.
To use them, insert the keys into the grooves in the stretcher bars located on the back inside corners of the canvas. Gently tap the keys into the stretcher bar grooves. Do not over tighten or it may cause damage.