Poly I zigzag so I don't melt the poly. wool I sometimes will hand stitch, sometimes fuse(depends on how densely I plan to quilt) and with cotton or cotton blend I use what I was taught was "French fuse", the woven, slightly stretching lightweight fusible that I buy by the yard and cut into 2" strips. It's a bit cheaper than buying the similar stuff as pre-cut roll for this purpose.