The stuff that I use is called "FOUNDATION". It is real thin and it is almost like Mohair...that is how thin it is...you can mark on it and it is sort of light grey/whitest color. A expert Quilter told me what the name of this is what I am using and it is called "FOUNDATION". You have to ask for it at JoAnn Fabrics. I love this method and it is so thin that you don't have any trouble sewing right through this stuff. That is how thin it is. I dearly love this stuff and the next time I go to JoAnn Fabrics I am going to get more of this. It does not add weight to your quilt pieces or your quilt at all. It is so thin that it does not add any weight at all.
Thanks for all of the lovely comments on my Triangle.