I think Prism is correct. When I quilted my last quilt, I decided to completely float the top. (Usually I attach the bottom of the top to the roller.) Each time I advanced the quilt, I carefully stitched straight down each side, from the previous side stitching, thinking that I was keeping the quilt nice and square. But when I got near the bottom I realized the quilt was getting smaller from side to side... I suddenly realized what was happening and had to do some work with the seam ripper (sometimes called skinning the quilt) to correct the problem. What was happening was that the quilting on the first pass made the quilt draw in, and I was starting the second pass with the edges aligned to that line, which resulted in the quilt getting a little narrower with each pass. I don't think I'll make that mistake again. The trick really is to measure with each advance, but I'm not convinced that even that is enough, because you still have to somehow make a straight line from the area that has been quilted and drawn up and the area that has not been quilted. I'm pretty sure the right solution is to stabilize the entire top (and not just by stitching around the edges) before starting any dense quilting. That's probably not as necessary with less dense quilting, or smaller quilts.