That thing is almost certainly based on a partlet, which is to say yeah, it's a dickie. It's probably nothing more than a rectangle of fabric gathered on a drawstring or ribbon around the neck and pinned, tied, or something similar under the arms to keep it in place under the outer garments. The gap is usually in the front but I think Catelyn Stark's is in the back most of the time.
(Been working on one myself, good luck with yours.)
EDIT: Also dagmarus, you're going to make a rad Eddard. Once you're done, can you mail me your head so I can do this shit up right?
ULTROS [Final Fantasy VI] (30%)
Haydee Tebelin (manga) [Gankutsuou] (10%)
Last edited by Mangochutney : 04-15-2013 at 02:37 PM.