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A pennyweight (dwt) is a unit of mass equal to 24 grains, 1 ⁄ 20 of a troy ounce, 1 ⁄ 240 of a troy pound, approximately 0.054857 avoirdupois ounce [1] and exactly 1.55517384 grams. [2] It is abbreviated dwt , d standing for denarius – (an ancient Roman coin), and later used as the symbol of an old British penny (see £sd ).
The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 pennyweights), and the troy pound (12 troy ounces). The troy grain is equal to the grain unit of the avoirdupois system, but the troy ounce is heavier than the avoirdupois ounce, and the troy pound is lighter than the avoirdupois pound. Legally, one troy ounce ...
English-speaking countries also used a system of units of fluid measure, or in modern terminology volume units, based on the apothecaries' system. Originally, the terms and symbols used to describe the volume measurements of liquids were the same as or similar to those used to describe weight measurements of solids [33] (for example, the pound by weight and the fluid pint were both referred to ...
Another difference arose when Britain abolished the troy pound (373.241 7216 g) on 1 January 1879, leaving only the troy ounce (31.103 4768 g) and its decimal subdivisions, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although they are no longer widely used.
Troy weight, avoirdupois weight, and apothecaries' weight are all built from the same basic unit, the grain, which is the same in all three systems. However, while each system has some overlap in the names of their units of measure (all have ounces and pounds), the relationship between the grain and these other units within each system varies.
The weight of the original sterling pennies was 22½ troy grains, or 32 "Tower grains". [ 26 ] : 116 Physical grain weights were made and sold commercially at least as late as the early 1900s, and took various forms, from squares of sheet metal to manufactured wire shapes and coin-like weights.
The penny led to the term "penny weight". 240 actual pennies (22.5 grains; minus the 1.5 grain for the seigniorage) weighed only 5,400 troy grains, known as a Saxon pound and later known as the tower pound, a unit used only by mints. The tower pound was abolished in the 16th century.
In the evolutionary approach, the foot was to be derived from these lengths by a simple integer factor, which would be either three (pendulum) or five (rod), i.e. lengthening it from the traditional value of about 304 mm by slightly over 1 inch to ca. 331 mm or shortening it by about a quarter of an inch to ca. 298 mm.