The main question is "how formal is a tartan dinner jacket? Is it less formal than a plain one or just as formal?" Now let me expand. I'm a practical man. Therefore, i hate suits(aka matched jacket and trousers). Usually, when I am supposed to wear a suit, I go with charcoal trousers w/ navy blazer and no one is the wiser. But now I really have to attend a black tie event. I'm thinking I should buy a tartan dinner jacket and tuxedo pants. But is a tartan dinner jacket less formal than a plain one? I would have no problem with buying a darker one(like a black watch dinner jacket). Maybe light ones are less formal, but how about dark ones?