Learn the rules, bend them until they just about break.

But, hey, they're all made up anyway so just keep a straight face.

These are pics from Geoffrrey (tailor) Kiltmaker's site. I'd go with that as the authority.

Here's a tweed formal enough for day events:


This is the Argyll, formal for evenings and most events:


This is the Prince Charles, think black tie and tux, this is formal:


Casual would be casual, or else it isn't casual. Flaming tongue Stones shirt and sandals works for me. Docs if I'm going to an event.