This interfacing consists, among other things, of the hairs of the horse’s tail, which were incorporated in the area of the weft threads. Hair canvas is used whenever a section of the garment (especially blazers or coats) needs to be reinforced without stiffening everything unnaturally.
Horsehair has characteristics that are desired in this case. It stabilizes the overlying fabric, but is still flexible.
The interlining is cut to size as desired and then ironed onto the wrong side of the fabric. The material is odourless.