Charlottes Fusible Web. Not many quilters & embroiderers are familiar with fusible thread.
I use it for machine applique. It looks a lot like dental floss. If you lay out a line of it and put your iron near it and steam it, the thread will shrivel up. It will bond fabric to fabric in a very thin line - the width of the thread. If you use a zigzag stitch, it gives more of a bonding area.