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Morning Dress is the English and American set of formal dress attire that is traditionally worn before nightfall. It is related to evening dress, which is usually worn after dark.

Morning dress attire consists of a few specific clothing articles. The most unique article is the jacket, whose design is derived from that of the frock coat, black or charcoal gray, with a long skirt in the back, cutting away to form an oblong tail as it suppresses the hips, and buttoning at mid-waist. Under the jacket is worn either a single- or double-breasted waistcoat, usually matching in color, and a white, stiff-collared shirt with a necktie. The trousers are almost as interesting as the jacket: They are striped in charcoal gray and black, and to be worn only with suspenders. Black dress shoes are a must, and, if it pleases, a cane and gloves can also be worn without the commission of any morning dress faux pas. You can even wear a gray or black top hat.

Morning dress, or, at the very least, the morning coat, was developed from the British equestrian tradition. In the mid-19th century, when modern morning dress was established, men might have worn frock coats on their morning horse ride, should the jackets have been suited to be worn so; but for the sake of ease, tailors began to modify frock coats to accomodate morning riders by gradually cutting away at the front panels of the jacket from behind -- forming the rear skirt. It's true, the rest of the attire carried over from morning riding dress, which was, until the mid-19th century, considered much less formal than evening dress; but with the new coat, the whole outfit became accepted by a more progressive fashion world as formal dress, which is just what it is today.

In the United States, the stroller jacket has overtaken the morning coat in popularity as formal morning dress. It is considered by most to be less formal than the morning coat, and looks something like a scant tuxedo jacket. It is also usually worn with a waistcoat, dress shirt and tie, but, unlike the morning coat, is often worn with solid-colored trousers, and the waistcoat is often white, or some color dissimilar to the jacket's.

References:
http://en.wikipedia.org/wiki/Morning_dress
http://www.lahacal.org/gentleman/morning.html