A tightening key is a small wedge or shim used in the construction of a canvas stretcher frame with expandable joints. The key is inserted into the slotted inside the stretcher bars at the mitered corners of the frame to prevent or adjust sagging.[1][2][3] Tightening keys are commonly made in plastic or wood, and are often called corner keys or corner wedges.[3] Tightening keys were introduced in the mid-18th century, making expandable (versus rigid) stretcher frames possible.[4]
Most stretcher frames have eight keys, two in each corner.[1] Each key is tapped in to expand the stretcher bars slightly, tensioning the canvas.[1][2] The keys can be used to re-tighten the canvas as it relaxes naturally over time.[5] If the key is tapped too forcefully, it may tear the side of the canvas.[4] Conversely, keys that become too loose can fall out during handling and damage the painting's surface.[1] The process of inserting keys into the stretcher is called "keying-out."[4]
Corner keys or wedges come in various forms. Most are manufactured for a specific size stretcher bar, making framers and artists hold various sizes in stock. A product called "wedgee" eliminates the need to stock many types as it has a male / female profile, which allows it to be pressed together to fit most corner cut-outs.[citation needed]
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