The London Lounge

So I always thought "Formal" meant white or black tie, dinner jacket/tux, etc., and that "Semi-Formal" implied a suit.

But, looking on wikipedia...

"A "formal" or white tie dress code typically means tail-coats for men and full-length evening dresses for women. "Semi-formal" has a much less precise definition but typically means an evening jacket and tie for men (known as black tie) and a dress for women."

Obviously that quote says Semi-Formal "has a much less precise definition," but, are there some loose rules of "Semi Formal" for

A. The US
B. The UK

?

And what about a blue SB or DB coat with khaki pants and a tie? Is that business-casual...even with the tie?

These are the correct traditional uses of the terms - wherever you are.

It is just that most people are so poorly dress educated they think:

1. formal=dinner jacket & semi-formal=lounge suit or sportcoat with or without tie
2. formal=lounge suit & semi-formal=business casual

Traditionally dinner jackets ('tuxedos') were worn at dinner - at home or out. They were often referred to as 'informal dinner clothes'. I strongly question whether they are formal dress at all.