Originally Posted by charity-crafter
Why does measuring the middle verses the edges work better for borders?
The center measurement seems to be the true measurement (or the top's potential). By the time the borders are added, the far edge may have gone wonky. So measuring the wonky outer edge will just make the final border wavy and the top not lay flat. If you cut/sew the final border to the center measurement, that length will adjust and sort of "force" the top back into submission. If it's too short, the border can be stretched just a bit. If it too long, the border can be eased just a bit.
I have heard people measure across the center and along both edges. In a perfect world, there shouldn't be a difference - or at least not much of a difference. If there is, some people figure out the average of the three measurements and use that as the border dimension.