I use cotton only for the fabric. For thread I use “machine quilting thread.” I just looked on the label to see what it’s made of and it’s all cotton. I use W&N and always thought it was 100% cotton until I read this post, lol. I prefer W&N but will also use other cotton/polyester blends. I’m not fussy about the thread, though I don’t want to use anything real cheap. I remember being told back in home-ec to always user mercerized thread. Seems it had some kind of coating on it to make it stronger.