Tightening key

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.

[3] Tightening keys were introduced in the mid-18th century, making expandable (versus rigid) stretcher frames possible.

[1] Each key is tapped in to expand the stretcher bars slightly, tensioning the canvas.

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.