Twist, Cup and Bow: Winding Sticks Make the Invisible Obvious

Lay a straightedge along the length of the board and look for light under it: that is BOW. Lay it across the width and do the same: that is CUP. Check in several places along and across, because a board is rarely uniform. Cup usually goes the same way on a flat-sawn board — away from the heart of the tree — which is why boards are often alternated in a glued panel. Both are straightforward to see and straightforward to plane out, and both cost thickness. Take the measurement before deciding the finished thickness, or you will find the board is thinner than the design needs after truing it.

Stand one stick across the board at each end and sight across their top edges from a low angle. If the board is flat, the two top edges line up as a single line. If it is twisted, one stick appears tilted relative to the other. The sticks amplify. They are much wider than the board is thick, so an end-to-end twist of a fraction of a millimetre becomes a visible several-millimetre offset between the two far corners — a difference you can see easily and could never feel. Make them from any straight stable stock, in a matched pair, dead parallel along their length. A dark inlay or a painted stripe along the top edge of one makes the sighting much easier, which is why traditional pairs have contrasting inlays.

Twist is removed by planing the two diagonally opposite HIGH corners down until the sticks read parallel. Mark the high corners in pencil while the sticks are still in place so you know which they are once you pick up the plane. Work diagonally across the board rather than along it — the high corners are on a diagonal, so that is the direction the shavings need to come from. Check again every few strokes. It is easy to overshoot and put a twist the other way in. Then re-check with the straightedge for bow and cup, because removing twist changes them. True the face completely before touching the edge; a face that goes out of true afterwards takes the edge's squareness with it.