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A pennyweight is an archaic unit of measurement which is still seen in certain applications, most usually in jewelry making. Pennyweights may be used for reference by other people who work with precious metals, such as dentists, who may opt to use them in dental fillings. The pennyweight has been replaced in most other applications by units of measurement such as those in the metric system, in the interest of using standardized measures.
This unit of weight is part of the troy measurement system which was inherited from the Romans, including the troy pound and troy ounce. A pennyweight under this system is equivalent to 1/20 of a troy ounce, or 24 grains. In the metric measurement system, it's roughly 1.5 grams, or .055 ounces in US customary units. People may note that precious metals such as gold and silver are sometimes sold in troy ounces, especially if they are in the form of minted coins and blocks, but the troy pound is no longer in use.
Historically, the weight of the penny in Britain was pegged to this unit, hence the origins of the name. Modern pennies weigh more, however, and since units of currency represent abstract values rather than being of literal value, weight is not as critical, although weights of coins are standardized for a variety of other reasons. Because coins no longer have intrinsic value, the use of precious metals for coin production has also been largely abandoned, as it is no longer necessary.
The pennyweight is abbreviated dwt, which might be a bit confusing, but is related to the earlier Roman term for this unit of measurement. Pennyweights are most commonly seen in use as a unit of measurement when people are talking about precious metals, although jewelers increasingly prefer units of measurement such as grams. Grams are viewed as more precise and reliable, and because they are widely used, they are a convenient method of measurement.
When a weight is given in pennyweight, a conversion is usually readily available. Experienced professionals who work with weights and measures regularly can sometimes even convert in their head for the convenience of customers. It can be helpful to convert to put weights into a system of measurement with which one is familiar. It is important, however, to make sure that conversions are done correctly, as when selling precious metals or when turning over pieces for repair work; making sure that they are weighed properly ensures that they are properly tracked and valued.