A chatelaine is a kind of fancy metal belt, usually worn as decoration rather than for any practical purpose. It is derived from the functional belt worn by the lady of the manor in olden days, from which she hung her keys and other tools such as scissors.
Chatelaine literally means 'lady of the castle.' Castles and manor houses typically kept many things under lock and key, not trusting the servants. The lady of the house would unlock the pantry, for instance, and give the cook or pantry maid what she needed to make the day's meals before locking the rest of the food away. Tea and sugar, being imported at great expense, were locked in separate chests, so the lady of the house needed a large number of keys to do her daily business.
Since the lady of the house was never seen without a chatelaine, this useful item became highly decorated and attractive; a chatelaine made of silver or gold could be seen as a sign of the wealth and importance of the owner. Eventually the function was dropped but a chatelaine remained as a graceful piece of jewelry meant to highlight the importance of the wearer.
A chatelaine is a very long, sweeping chain, often made of flat rectangles of decorated metal linked together with chain links. The chain links around the wearer's waist, with one end dangling down the front of the dress to the floor. Chatelaines can be jeweled or enameled, or simply made of precious metal. They go very well with a Celtic outfit.
Today, a chatelaine is usually reserved for wearing with Renaissance Faire garb. If going to the faire as a high-born lady rather than a woman of the lower serving classes, a chatelaine is a good mark of one's class.
A chatelaine is a little harder to work into a modern day wardrobe, where its sweep might interfere with activities such as riding a motorcycle, for example. Shorter, hip-length chatelaines work well with slacks, and can even be returned to their earlier function -- carrying your keys.