Typically this would be your "side seam to side seam" measurement around the small of your back or the largest part of your hips/buttocks.
It's useful when constructing a kilt for somebody with a belly -- if you just measure around the waist and around the hips, you may get two numbers that are equal or even "my hips are smaller than my waist." But the front apron needs to hang straight down from the belly, not pull in below it. So splitting the measurements in two, as if you were wearing a pair of trousers and measuring from one side seam to the other, can help you come up with waist and hip numbers that result in a well-fitting kilt.
Hope this helps!
Proudly Duncan [maternal], MacDonald and MacDaniel [paternal].
Bookmarks