Also known as a Canvas Wedge or Painter's Corner. They are used to keep the frame pushed apart to hold the canvas taught. There should be 8 of them, two for each corner.
The edge of the frame is made up of 4 stretcher bars with joints in each corner.
Looking from the inside, each corner will have grooves cut into it at an angle. They will be offset on each stretcher bar.
Tap the frame apart before putting in the wedges.
Use a hammer to tap on the stretcher bars from the inside. On smaller frames this can be in the middle to evenly do each edge, on larger frames it may be necessary to tap nearer the corners and use the same force at either end of the same stretcher bar.
Tap on the inside of each edge, so that the canvas is stretched evenly. The amount of movement is likely to be less than a millimetre and may not even be visible at the corners, but will be enough to tighten the canvas.
Be careful with existing paintings not to overstretch the work and crack the paint.
Tap in the wedges
The wedges are to hold the joint slightly apart.
They can be fitted parallel to the frame. My preference is to align them pointing towards the centre of the canvas. In either case, the longer edge is closest to the outside of the frame. I find the later method easier to get the hammer in to hit the ends of each wedge as they do not overlap.
Whichever position they are fitted, simply tap them in. The wedge shape will stop the corner closing up and can be used to fractionally force the frame apart to get that last bit of stretch needed to make the canvas evenly taught. The wedge in one stretcher is pushing against the timber of the other stretcher.
Put one wedge in the ends of each stretcher, so each corner will have two wedges.
They stay there
Once fitted the wedges should remain there. If the canvas sags, the frame can be tapped out a bit and the wedges tapped in again to hold it in place.
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There are a number of articles by others that go into more detail. I found the following clear and comprehensive:
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