I don't have the 770 anymore, but I think it's the one that looks like an underline. I use iron on on the back, then peel off after it's finished. I use solvy on the top. It's been awhile since I embroidered t shirts, what you have to watch for most with them is stretching. You wouldn't want the fabric stretched during hooping, just taught. Hope that makes sense. The solvy on the top will keep the stiches from embedding into the fabric, thus they will stand up better.