The rules of “white tie” attire. This article is a bit snarky, but informative.
‘The trousers are a similar cut to chinos. I suspect they’re not high-waisted, so he’s had to get an extra-long waistcoat to cover the top of the trousers. So now you’ve got at least four inches of white showing between the coat and the trousers. Look at Prince Charles. With proper military fashion, you don’t see any white in between, and the coat is just covering the waistcoat.’