Petersham ribbon, also called Petersham facing or simply Petersham, is a thick, stiff, flexible corded ribbon and used as facing by milliners and tailors. It is woven so that once steamed, it will take on and support a particular curve of fabric. This makes it useful for obtaining a smooth edge on the brim of a hat, for example, without forming puckers or wrinkles which would result from the use of traditional flat ribbon or other flat fabric.
It is also useful as an alternative to bias tape for making fabric conform closely to the shape of the body wearing it in a corset, for example, or along the waistline of a pair of trousers or a skirt. Petersham is very similar to grosgrain ribbon in appearance: both have closely spaced horizontal ridges, but Petersham has a flexible picot edge allowing it to be shaped with an iron, whereas grosgrain cannot be shaped this way.