A wood caul, with its slightly bowed face down, helps distribute clamping pressure to the middle of a long workpiece such as a face-frame rail.

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A wood caul, with its slightly bowed face down, helps distribute clamping pressure to the middle of a long workpiece such as a face-frame rail. To get the same effect without having to make bowed cauls, use a straight piece of hardwood or aluminum channel for the caul. Then, insert crafts sticks under the caul where you need a little extra downward pressure.
—Fraser MacLean, Langley, B.C.