Step Ins?

When I was a kid my mother had a mysterious garment called Step ins which I never quite got to see and this has worried me ever since. What on earth were they? I think they were some sort of undergarment but they can't have been ordinary undies. They must have been something that you arranged on the floor first and then stepped into them before hoisting them up. Maybe it was a thing made of elastic with little clothes peg things around it that you clipped your stockings on. Does anybody know?