Open Robes

Hmm ... after going through the Gallery of Fashion, I'm starting to think that those silk open robes were worn with a petticoat.  I had been under the impression that open robes were worn over muslin gowns as a way of dressing them up for the evening - maybe that's a re-enactorism?  I can't think where I've seen it, now that I try to remember, but I guess I'm not doing an open robe as part of my thesis.