Parallel Clamps

A parallel clamp builds the same sliding-jaw idea as an F-clamp around a rectangular, not round, beam, with jaw faces engineered to stay parallel to the workpiece as they tighten instead of pivoting and digging in at an angle the way a round-bar clamp's jaws can under uneven pressure. The result is even clamping pressure across the whole jaw face, and the clamp stands on its own flat jaw on a bench rather than rolling -- both genuinely useful during a panel glue-up with several clamps going on at once.

That refinement costs real money -- a parallel clamp runs well above an equivalent-length pipe clamp or basic F-clamp -- which is why most shops build up a set gradually rather than replacing everything at once.