a bracing system for canvas stretcher frames allows for positioning of one or more braces in a stretcher bar frame , to prevent inward pulling of the stretcher bars under canvas tension . the braces are modular in design and allow convenient assembly of multiple - braced stretcher bar frames . the braces have tenons which engage with a groove extending around the inner periphery of the stretcher bar frame , and the braces themselves preferably have grooves in their edges , to receive the tenons of further braces .