Foot median

The foot median runs from the head down to the exact middle of the foot. On a flat drawing that sounds redundant — but a spinnaker is not flat. Because the sail is a curved surface, you can hold the luff, leech and foot lengths fixed and still build two noticeably different sails. The foot median pins down the rest.

You will meet it in Dutch class rules as the onderlijkzwaartelijn, and it appears in almost every klassenvoorschrift alongside the widths at half and three-quarter height. Those four numbers together are what actually lock a one-design sail's shape.

It is a measurement over the cloth, not through the air: laid flat, from corner to midpoint. See our Dutch class dimensions for real examples.