Because Britain, for various historical reasons, has such a disproportionately large impact on masculine sartorial conventions, tailors usually refer to vests in the British manner as waistcoats. If you are really old-money, you will pronounce it as “weskit.” Lexical matters aside, a non-matching waistcoat without a jacket is now a popular and accepted part of casual wear. It’s a great way to dress up jeans. (I am particularly keen on tweed waistcoats: Nothing says “intellectual interests” like that one small garment.)

Such an outfit is appropriate for anywhere but those places where one would wear a business suit. The pieces of a suit, on the other hand, are meant to be worn all together. There are no public places in which a guy wearing a suit should remove his jacket ( especially in a restaurant. Keep it on; don’t hang it on the back of your chair). This goes for two-piece and three-piece suits. The waistcoat of a suit is not meant to be seen from behind: That’s why the back of it is usually made from satin. And walking around with your jacket off and your suspenders on display is like walking around in your underwear.