A tailcoat is the most formal dress that a man can wear and is always black or midnight blue. A tailcoat is worn during events that start after 6 pm, e.g. galas, formal receptions and big evening events. Weddings, academic or diplomatic events are times when a tailcoat can be worn during the day. A tailcoat can only be replaced by a dark suit, full dress uniform or folk costume.
A tailcoat jacket features elongated tails and silk covered lapels. Trousers belonging to a tailcoat can be identified by the double-braided silk running down the sideseam. A white marcella vest is usually worn with tailcoats, the use of black vests is usually notified in the invitation to the event (Events as such can be e.g. Academic daytime events.) Contrary to what you see a lot of these days, the length of the vest should not exceed the length of the jacket.
The shirt that is worn with a tailcoat is made of white cotton with a starched piqué bib front and single cuffs that are worn with cufflinks. The choice for a bowtie is a white cotton piqué and the pocket square is white and can be silk or linen. If orders or a bouttoniere is worn, a pocket square shall not be worn. If miniature orders are worn however a pocket square is allowed.
The shoes should be black patent leather oxfords but highly polished black calf oxfords are also acceptable. The only accpeted socks are black silk ones. The overcoat that is worn should cover the tails of the tailcoat and be dark in colour.