This is a true radius TDL IG spiderweb sash. True radius – because it’s a true half circle. TDL stands for “true divided light” which indicates the glass is REALLY divided (not just pieces of wood clued on one piece of glass) and IG means “insulated glass”. Two pieces of glass are bonded with an air space between. Energy efficient and true craftsmanship. And we guess we don’t have to explain why it’s called “spiderweb” sash. 😉 Wouldn’t this look amazing on your house?