A pennyweight is a unit of mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 that is equal to 24 grains
Grain (measure)
A grain is a unit of measurement of mass that is nominally based upon the mass of a single seed of a cereal. From the Bronze Age into the Renaissance the average masses of wheat and barley grains were part of the legal definition of units of mass. However, there is no evidence of any country ever...

, 1/20 of a troy ounce
Troy ounce
The troy ounce is a unit of imperial measure. In the present day it is most commonly used to gauge the weight of precious metals. One troy ounce is nowadays defined as exactly 0.0311034768 kg = 31.1034768 g. There are approximately 32.1507466 troy oz in 1 kg...

, 1/240 of a troy
Troy weight
Troy weight is a system of units of mass customarily used for precious metals, gemstones, and black powder.There are 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system. The troy ounce is 480 grains, compared with the avoirdupois ounce,...

 pound, approximately 0.054857 avoirdupois ounce
The ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems...

  and exactly 1.55517384 gram
The gram is a metric system unit of mass....


In the Middle Ages, a British penny
British One Penny coin (pre-decimal)
The English Penny, originally a coin of 1.3 to 1.5 g pure silver, includes the penny introduced around the year 785 by King Offa of Mercia. However, his coins were similar in size and weight to the continental deniers of the period, and to the Anglo-Saxon sceats which had gone before it, which were...

's weight was literally, as well as monetarily, 1/20th of an ounce and 1/240 of a pound of sterling silver
Sterling silver
Sterling silver is an alloy of silver containing 92.5% by mass of silver and 7.5% by mass of other metals, usually copper. The sterling silver standard has a minimum millesimal fineness of 925....

. At the time, the pound in use was the Tower pound. 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.

The troy pound and the pennyweight lost their official status in the United Kingdom of Great Britain and Ireland
United Kingdom of Great Britain and Ireland
The United Kingdom of Great Britain and Ireland was the formal name of the United Kingdom during the period when what is now the Republic of Ireland formed a part of it....

 in the Weights and Measures Act
Weights and Measures Act
A Weights and Measures Act is an Act of Parliament determining trade law where the weight or size of the goods being traded are important. For example, if a bottle of milk is for sale and has a label saying it contains one pint, then the law states that it must contain that amount.-United...

 of 1878. Only the troy ounce and its decimal subdivisions remained official after the 1878 act. The troy ounce enjoys a specific legal exemption from metrication
Metrication refers to the introduction and use of the SI metric system, the international standard for physical measurements. This has involved a long process of independent and systematic conversions of countries from various local systems of weights and measures. Metrication began in France in...

 in the UK.

The pennyweight is the common weight used in the valuation and measurement of precious metals. Jewelers use the pennyweight in calculating the amount and cost of precious metals used in fabricating or casting jewelry. 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
In the Roman currency system, the denarius was a small silver coin first minted in 211 BC. It was the most common coin produced for circulation but was slowly debased until its replacement by the antoninianus...

, was the abbreviation for penny before decimalization of the British monetary system. Alternate abbreviations are pwt and PW.

Although the abbreviations are the same, the pennyweight bears no relation to the actual weight of the penny nail, but to the price for a hundred nails in 15th century England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

: the larger the nail, the higher the cost per hundred.

The pennyweight also bears no relation to the weight of current penny coins. The weight of a US penny minted after 1982, for example, is 2.5 grams.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.