In modern times, unless it's really formal, I wouldn't wear a fly plaid. In fact, I have never purchased one, because even for my formal mess dinners, I'd never wear it.

"The plaid (plaid*) was the leftover top half of fabric not used for the short kilt. Men continued to wear this with the kilt as a separate garment or cloak in various fashions which are shown in the picture section. It was worn with trews and jacket as well , laid across one's shoulder. When some men took up European fashion of plain wool breeches, waistcoat and jacket, they continued to carry their tartan plaid over their shoulder. Today, we find the plaid still in use, most commonly by pipe bands."


A thought: Is it possible, that the Scots always wore the philibeg, but with a fly as a blanket, so it appeared that it was a "great kilt" (philamore or breacan feile)? From what I've read, no one has actually found a great kilt, and they're only going on what few photos of the era have been found. I think I read, that a kilt was found in a bog of Scotland? Does one photo which appears to show one large piece of tartan, mean that no one wore a smaller-fabriced kilt?