Its the quilting threads, not the seams that I made to put the blocks together. All I did was tug it a little to cover myself & I heard the thread break. It's happened twice on the same quilt & I saw broken threads on another quilt that I made for 1 of my grandsons. I did some stitch on the ditch quilting & those are the threads that I hear break. The threads that I use are new, so I didn't think that it oucld be the thread. Only recently did I realize that thread size in "free motion" quilting made a difference or not. Sadly, I bought some of my quilting thread at Walmart, without looking at the size. It just read "quilting thread". Looking at them now, some say 30W & some 50W, Machine quilting & craft. The brand is STAR. by Coats & Clark.