A tightening key is a small wedge or shim used in the construction of a canvasstretcher 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]