My current fascination with haps has a lot to do with how the haps are constructed. Houlland is put together by first knitting the bottom border back and forth to make a long skinny rectangle.
Then the shawl is knit upward from stitches picked up along the top of the border.
The triangular shape is formed by decreases on each end of every right-side row. These decreases have the effect of pulling the long bottom border up to form two sides of the triangle.