Underwire casing is a tube of plush fabric that has a channel for underwire to slide through. It is sewn to the bra to hold the wire next to the body. This white colored nylon underwire channeling is thin with a soft plush back for comfort against the skin and has a great ability to curve around your cup. The front side is thinner, more of a ribbon look for a thinner profile when used. The width measures 3/8 inch wide and is 100% nylon. The underwire casing does not stretch, but it is flexible for sewing into curves.