Single fold has the two outer edges folded in, and then it is folded in half.
The double thickness is only where you sew it onto either side of the quilt. The edge that is on the edge of the quilt, is actually only a single thickness.
Single fold (one thickness) bias tape... the name is misleading
http://www.google.com/images?rlz=1T4...w=1003&bih=498