Wikipedia
A stretcher bar is used to construct a wooden stretcher used by artists to mount their canvases.
They are traditionally a wooden framework support on which an artist fastens a piece of canvas. They are also used for small-scale embroidery to provide steady tension, affixing the edges of the fabric with push-pins or a staple gun before beginning to sew, and then removing it from the stretcher when the work is complete. Stretchers are usually in the shape of a rectangle, although shaped canvases are also possible.
Usage examples of "stretcher bar".
Bristling from the side of it at a ninety-degree angle was the one-inch stretcher bar that had connected it to the other rear leg.
With one leg broken, another cracked, and the stretcher bar splintered, the chair wasn't as formidable a weapon as it had been.