Proportion: lofting arcs. The red band is drawn by lofting between two arcs.
It takes three points to define an arc: two to cut a slice of pie, and one to choose between the slice and the rest of the pie. The circle going around the outside of the figure shows the position and effect of the third point.
Lofting arcs can give strange results. The blue blob in the middle demonstrates lofts of two arcs that line up poorly.
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