Winding stick

It is common for a woodworker to make a matching pair for the purpose, and contrasting colors of woods improve the discernability of differences in level between the two sticks as they are compared.

The woodworker then stands back a short distance and sights across the top of the two sticks.

They are not dependent on being the same height along their length; the critical element is that they are parallel to each other and then used in that configuration.

Firstly the soles are checked to be flat; a long shooting board is recommended for the job.

Next align both sticks either back to back or face to face, whichever is easiest to secure, then take light passes over the tops with a hand plane until even shavings are produced from both sticks.

Winding sticks in use.
Sticks are used facing one another (top). When aligned for planing, they can be placed back to back (bottom left) or face to face (bottom right).