For perfect miter joints, it’s more important that both miter cuts add up to precisely 90° than each miter is exactly 45°.

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For perfect miter joints, it's more important that both miter cuts add up to precisely 90° than each miter is exactly 45°. This blade-straddling miter-cutting sled ensures that those complementary cuts always equal a right angle. To install the miter-slot guides on the bottom of the sled, place the guides in your saw's slots, set the sled on top of them, and then drive screws into the guides through the top of the sled. Now cut a blade kerf about halfway across the sled. Using a combination square, mark the location of the right-hand fence 45° to that kerf, and install the fence on that line. Now place a reliable framing square against that fence to locate the left fence, rest the left fence against the square, and then screw it into place. Make test cuts and if needed, remove the fence screw farthest from the kerf, adjust the fence's angle, and drive a new screw to secure it.