Morning Coat

Two men wearing morning coats at a wedding. 1929.
The modern morning coat (or cutaway in American English) is a man's coat worn as the principal item in morning dress. The name derives from morning nineteenth century horseback riding exercise for gentlemen. It was regarded as a casual form of half dress. Gradually it became acceptable as an alternative to the frock coat for formal day wear or full dress. Since the nineteenth century it is normally only seen at weddings, formal baptisms, in England, races such as Royal Ascot and the Derby when it is worn with a contrasting waistcoat, usually light grey or sometimes 'fancy'. It is very occasionally seen at funerals but more often it is used as day wear at formal luncheons, especially civic occasions under formal gowns, when worn with a black matching waistcoat (US 'vest'). Male members of the cabinet of Japan wear it in their first public appearance following the formation of the cabinet.
The Marshall and Clerk of the United States Supreme Court wear morning coats when the justices are appearing in public wearing their traditional robes, for example when the court is in session, or when attending the President's State of the Union address. At one time all attorneys appearing before the court wore morning coats but now wear standard business attire. The United States Solicitor General (when the office is held by a male) and his or her male deputies continue the tradition of wearing morning dress when arguing before the court.[3]
A morning coat is a single-breasted coat, the front parts usually meeting at one button in the middle, and curving away gradually into a pair of tails behind, topped by two ornamental buttons on the waist seam. The lapels are usually pointed (American English peak), not step (notch), since the coat is now only worn as formalwear; when it was first introduced, the step lapel was common, since it was worn as half dress. The coat can be grey or black as part of morning dress, and is usually worn with striped, or very occasionally checked, trousers.
The morning coat may also be worn as part of a morning suit, which is mid-grey with matching trousers and waistcoat.
During the Victorian and Edwardian era, in America morning coat referred to a single-breasted frock coat, so the British then made fun of the fact that Americans were unable to distinguish between morning coats and frock coats. In modern American English, morning coats are referred to as cutaway coats.

from:

http://en.wikipedia.org/wiki/Morning_coat#Morning_coat