I have used whatever I have on hand. I have used coffee filters as a paper foundation (then eventually cut square), and I also use my "ugly" fabric that I don't want to use for anything visible! like the others said, I'm sure the foundation is due to the bias. I find that using the foundation is much faster though. I feel more freedom for not having to perfectly line things up. but then again, I do my strings kinda wonky to get that advantage, too.