Cauls keep glue-ups flat and flush
As you squeeze boards together with pipe clamps, they sometimes arch or slip out of alignment. Pairs of upper and lower cauls are the solution. I lightly squeeze the cauls with bar clamps, then tighten the pipe clamps, then tighten the cauls a bit more… I continue this back-and-forth process until the boards are joined flush and flat.
My favorite cauls are made from 2x4s. I carefully select ones that have a slight bend or “crown” along the 1-1/2-in. edge, but no twist or warp. The crown is an advantage because it creates extra pressure in the middle of the caul. I label all my cauls with an arrow marking the direction of the crown and the length of the caul.
What’s a caul?
If it’s designed to spread clamping pressure over a wide area, you can call it a caul.