Corner Key is a small wedge or block of board, timber or plastic, inserted into the slotted inside corners of a canvas stretcher frame to prevent or adjust sagging.
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Use
Sometimes called a tension key, the piece is tapped in to expand the stretcher bars slightly, tensioning the canvas. The keys can be used to re-tighten the canvas as it relaxes naturally over time. Tightening keys are commonly made in plastic or wood, and are commonly known as corner keys or corner wedges.
Types
Corner Keys or corner wedges come in various forms. Most wedges are available for a single size stretcher bar, making framers and artist hold various sizes in stock. A product called "wedgee" eliminates the need to hold many types of corner keys in stock as the wedgee has a male / female profile which allows them to be pressed together to fit most corner cut outs.