A rat's nest on the bottom means 1) the upper tension is not right or it is not threaded correctly and/or 2) you didn't hold your threads when you started a seam. Since I have used a thread starter -- which is a very small piece of fabric in which I take the first four stitches, then sew on air for a stitch or two, then sew my real seam while still connnected to my thread starter -- I have not had one rat's nest. Not one. Before that, I had a lot of them.