I LOVE the prewounds..they hold so much more thread and it is finer weight for the back of the stitchout. I have Bro machines and my ULT's use the "L" size. The class 15 are the same diameter, but taller & won't work in my machines. I buy the plastic bobbins from metroemb and can re-use when I put color threads in the bobbin. I think the "M" ones are for commercial machines. I will never wind for embroidery again--except for matching colors. You will be amazed at how long you can embroider without replacing a bobbin!