I used to wind thread on top of thread. But, when you come to the end of the top layer of thread, there's a tangle -- no big deal, right? Then I became a sewing machine mechanic. Now, I know what a tangle of thread can do to a sewing machine. When thread tangles, it pulls the needle into the hook and damages it. Tangles can also pull the needle into the needle plate, break the needle, and even knock the timing out. So I never wind thread on top of thread any more. It's just a little thing I do to take care of my sewing machines. I pull the thread off and put it in a jar, and it adds a spot of color to my sewing room. I currently work with about 30 bobbins -- 10 for polyester threads and 20 for cotton threads.