Medium weight iron-on (fusible) interfacing by Vilene Vlieseline.
15cm x 20 cm piece
easy to use: simply place over the back of your embroidery (shiny-side down), layer over a damp cloth, and iron on!
Paraffle Note: this interfacing is great for reinforcing pieces of embroidery, to make them stiffer and more durable. I always use this interfacing on the inside of my embroidered tote bags, too - it fuses to the threads and stops them from tugging while the bags are being used!