<P> The US has not adopted the metric system despite many efforts to do so, and the pound remains widely used as one of the key United States customary units . </P> <P> Historically, in different parts of the world, at different points in time, and for different applications, the pound (or its translation) has referred to broadly similar but not identical standards of mass or force . </P> <P> The libra (Latin for "scales / balance") is an ancient Roman unit of mass that was equivalent to approximately 328.9 grams . It was divided into 12 unciae (singular: uncia), or ounces . The libra is the origin of the abbreviation for pound, "lb". </P> <P> A number of different definitions of the pound have historically been used in Britain . Amongst these were the avoirdupois pound and the obsolete Tower, merchant's and London pounds . Troy pounds and ounces remain in use only for the weight of certain precious metals, especially in the trade; these are normally quoted just in ounces (e.g. "500 ounces") and, when the type of ounce is not explicitly stated, the troy system is assumed . </P>

Why is the short form of pounds lbs