Stretcher

Wooden bars, usually made of pine, that constitute a frame over which the canvas of a painting is stretched. Although stretchers can be any shape, most are rectangular. The edge of the canvas is attached to the stretcher with tacks, or with staples driven by a staplegun. The size of a stretcher can be changed slightly through the adjustment of keys (the small wedges visible in this example) or with expansion bolts at the four corners. A thin molding (3/4-round) should be attached to the outer edges of the stretcher, in order to keep the canvas from touching any surface other than those edges. (Artlex.com)

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