Measurements & numbers
Mid-girth
The width of a sail at half its height - the number that defines a downwind sail.
DutchHalve breedte
Mid-girth is the sail's width halfway up, given as a percentage of the foot length. It is the single most useful number in the downwind world, because of one piece of geometry: a triangle with its apex at the head is exactly 50% wide at half height. So everything above 50% is bulge - actual, physical fullness.
That is what the code numbers mean. A Code 50 is barely wider than a triangle: flat, close-winded, almost a reaching sail. A Code 70 has real shape. A symmetric spinnaker at about 90% is a balloon.
Compare the two diagrams on this page: same rule, same geometry, wildly different sails. Every sail diagram on this site is drawn from its mid-girth, which is why they can't drift out of step with each other.