I specify Brits because it was a Brit who brought it to my attention, but of course anyone who recognizes what this is please respond. This Brit was teaching a workshop in book binding. We used it instead of mull for stabilizing the book spine, and I found I prefer it, but I can’t seem to find it by that name. He’s the one who spelled it out that way. He said it was also called by another name (which I neglected to write down) and that it had sometimes been used on its own to make things like christening outfits for babies. Unfortunately, I don’t know how to reach him and he is back in the UK by now. When I called it that at a US fabric store (Joann) they had never heard of it. My little scrap, about 2x3.5", weights about half a gram. My micrometer measures the thickness as 0.25 mm.
Description: tightly woven, might be cotton-poly mix, it has no give in either direction, very crisp, as if it had been starched or sized or something. Since I mentioned interfacing, I will mention that it would be the sew-in kind, not the fusible kind. It seems to be similar to Pellon Shir-Tailer but I could only find that with the fusible glue on it.