I don't really care if it is cotton, poly, nylon, or rayon. If I like it and it works on my machine I'll use it. I have a lot of Aurifil and Connecting Threads Essentials I'm using now. Both brands have cotton and poly threads, doesn't matter to me when I'm piecing. I use the prettier color thread for machine quilting or decorative stitches. Any good thread will last in a quilt much longer then I'll be alive so why stress over it?