Construction
Since a stretcher is simply a frame, it 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. Corner supports can also be made using simple woodworking techniques, however.
The mitre 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 re-tensioned later, as it has a natural tendency to stretch and sag over time. In contrast, strainer bars stretch canvas in a fixed (non-adjustable) way.
Read more about this topic: Stretcher Bar
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