A pennyweight (abbreviated dwt or denarius weight) is a unit of mass that is equal to 24 grains, 1⁄20 of a troy ounce, 1⁄240 of a troy pound, approximately 0.054857 avoirdupois ounce and exactly 1.55517384 grams.
Contents
History
In the Middle Ages, a British penny's weight was literally, as well as monetarily, 1⁄20 of an ounce and 1⁄240 of a pound of sterling silver. At that time, the pound in use was the Tower pound (5,400 troy grains). The medieval English pennyweight was thus equal to 32 Tower grains (also known as wheat grains). When Troy weights replaced Tower weights in 1527, the Troy weights were defined in such a way that the old Tower pound came out to exactly 5400 Troy grains (also known as barleycorns), the Tower pennyweight 22 1⁄2 Troy grains (and thus approximately 1.46 grams). After 1527, the English pennyweight was the Troy pennyweight.
Usage
The Troy pound and the pennyweight lost their official status in the United Kingdom in the Weights and Measures Act of 1878; only the Troy ounce and its decimal subdivisions remained official. The Troy ounce enjoys a specific legal exemption from metrication in the UK.
The pennyweight is the common weight used in the valuation and measurement of precious metals. Jewellers use the pennyweight in calculating the amount and cost of precious metals used in fabricating or casting jewellery. Similarly, dentists and dental labs still use the pennyweight as the measure of precious metals in dental crowns and inlays.
The most common abbreviation for pennyweight is dwt; d, for the Roman denarius, was the abbreviation for penny before Decimalization of the British monetary system. Alternate abbreviations are pwt and PW.
Uses unrelated to weight
Although the abbreviations are the same, the pennyweight bears no relation to the weight of the American penny nail. That name is derived from the price for a hundred nails in 15th century England: the larger the nail, the higher the cost per hundred. The pennyweight also bears no relation to the weight of the American "penny" (1 cent) coin, which weighs 2.5 g (for those minted after 1982).