If you are restoring a 65 - 70 Mustang, this replacement panel welds to the inner rocker panel to replace rusty or dented rocker panels. It fits both fastback/coupes and convertibles (fastback/coupes have a different inner panel to a convertible). Sometimes called the sill (or door sill), it fits underneath the door from the front fender to the rear outer wheel house. This part is made from a quality metal and is an excellent copy of the original. It fits on to the existing parts without having to trim or bend anything.
Rocker for 1965 Coupe was too wide and I will have to cut it down or find a better one. Very disappointed. The panel measured 6 3/4 inches when needed to be 5 3/4 tall. Not only that, but the bend between the flange against the inner rocker and the sill plate where you pilar rest is too wide by 3/8 of an inch. Very poor fit.