The finest frames have the wooden wedges in a stretched canvas secured to prevent them dropping out in transit or as the wood dries.
If they do drop out, they often fall between the canvas and the stretcher causing a bump to appear on the surface of the canvas. FATG GCF standards advise using a method to hold the wooden wedges in place.
These very neat wedge retainers are quickly fixed with a brass pin (included) and give a very professional look to the back of the frame. They allow the wedges to be tapped in further as required.