Why they are called sails on a windmill

The board above explains that windmills were turned by a wooden framework over which a sail was spread. In very windy conditions, you could wrap up the sail so the wind was catching less of an area and so providing a smaller force. I like the solution with spring-loaded shutters. A stronger wind puts a bigger force on the spring, opening the shutter and letting some of the wind straight through.

Notice that the windmill has been done with two of each. And notice that they are diagonally opposite each other. The spring loaded sails will adjust to wind strength automatically whilst a human has to make a decision on the canvas sails. This could lead to uneven torques if different types are paired up. I suppose that the diagonal pairings act as a couple in the technical sense.