I use a 40 or 50 weight on top. I've even used a 30 weight once--Sulky Blendables. In the bobbin, I prefer the 60 weight poly pre-wounds. The weight has something to do with how much the thread would weigh if stretched out over a mile, I think. I go buy how bulky it appears to my eyes. C&C All Purpose thread is supposedly a 50 weight but if you compare it to their 50 weight cotton machine quilting thread--it seems much thinner. I guess the cotton wrapped poly core weighs more than a triple cotton strand. And I'm not going to stretch it out over a mile to weight it--LOL.