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A stretcher bar is a wooden frame used by artists to mount their paintings. They are traditionally a wooden 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.

Since a stretcher is simply a frame, they can be constructed in a variety of ways. The differences in construction have to do with how the corners are built. Commercially available pre-fabricated stretchers come in segments with interlocking corners, that can be fit together like puzzle pieces. You can build your own corner supports using simple woodworking techniques, however. The miter joint is the most popular method of adhering corners on a stretcher, although butt joints are also fine if used in conjunction with gussets. Keys or small triangle wedges are inserted in the joint after stretching the canvas to give the canvas its final tension. When fastening the canvas, pressure should be distributed evenly around the stretcher to minimize warping due to unequal distribution of pull.

Unlike other types of frames, the corner joints in stretcher bars are not glued or fastened in any permanent way. This allows the canvas to be retensioned over time, as it has a natural tendency to stretch and sag over time. In contrast, strainer bars stretch canvas in a fixed (non-adjustable) way.

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