Dirks are customary for civilian wear. This originated with the Victorian "everything, PLUS the kitchen sink" look. "Traditonal" US usage (in the _So you're going to wear the kilt_ sense) is for black tie equivalent.

I wear mine for full formal occasions when I'm not wearing a waistcoat. (It's hard to wear it with a waistcoat, since there's no belt to hang it on. It says something about a culture when a knife with an 11-inch blade is considered appropriate for wear to a formal dinner.) I also wear mine to Renfaire, with period shirt and philabeg. I don't wear it with a tweed jacket at the highland games, though there are people I consider sane who do. There are also people who wear a dirk with t-shirt and sneakers at the games, who I do NOT consider sane.

Will Pratt