Winding Sticks

Winding sticks are a pair of straight, parallel sticks (traditionally contrasting in color, or with a light-colored strip inlaid on one) laid across a board at each end, perpendicular to its length. Sighting across the tops of both sticks from one end of the board reveals any twist (also called "wind") in the board -- if the two sticks don't appear parallel to each other from that sightline, the board isn't flat across its length, even if it looks flat from directly above.

It's a simple, essentially free tool -- any two identical straight-edged scraps work -- but it does something a single straightedge laid flat on the board can't: a straightedge shows cupping or bowing along its own length, but twist only becomes visible when you're comparing two reference points at a distance, sighted down like a rifle barrel.