Most major brands produce bobbin thread and mark the spool as such. I never buy it. I always use a 30 weight cotton to piece, and anything I like that matches what quilt design I want. I don't pay attention to weight, but I do wind the bobbin with the same thread I'm going to quilt with. I mostly use rayon or polyester, but there are many brands who produce cotton for machine piecing (it's not waxed like most hand quilting thread). Even then I use the same cotton thread in my bobbin as I quilt the top with. I do make sure the bobbin thread will match my backing color. I'm not sure where the "quilt with cotton" thread on cotton fabric got started--I heard it years ago when I first started quilting. Boy would my Holiday pieces look dull if I'd stuck to that rule! Worst of all, I've resorted to using any bobbin at hand that had the color I needed, I've even put the bobbin on the upper spool when necessary. Hope the quilt police don't knock on my door.