Caul

A caul is a rigid, flat board (often with a very slight curve deliberately built in) placed between a clamp and the actual workpiece during glue-up, spreading the clamp's pressure out over a wider area instead of concentrating it at the clamp's own small contact point. Without cauls, a series of clamps across a wide panel glue-up can leave visible ridges or unevenly-pressed glue lines right at each individual clamp location, since the clamping pressure is only really strong directly under each clamp head.

A slightly convex caul (curved so its center touches the work first) is a deliberate refinement -- as clamping pressure increases, the caul flattens out and presses evenly across its whole length, rather than a perfectly flat caul, which can leave the panel's very center slightly underclamped compared to the ends.