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. Tightening keys are commonly made in plastic or wood, and are often called corner keys or corner wedges. Tightening keys were introduced in the mid-18th century, making expandable (versus rigid) stretcher frames possible.
