The imperial and US customary measurement systems are both derived from an earlier English system of measurement which in turn can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure. The US Customary system of units was developed and used in the United States after the American Revolution, based on a subset of the English units used in the Thirteen Colonies. It is the predominant system of units in the United States and its territories, except for Puerto Rico and Guam, where the metric system was introduced under Spanish rule and remains prevalent. The imperial system of units was developed and used in the United Kingdom and its empire beginning in 1824. The metric system has, to varying degrees, replaced the imperial system in the countries that once used it. Most of the units of measure have been adapted in one way or another since the Norman Conquest (1066). The units of linear measure have changed the least – the yard (which replaced the ell) and the chain were measures derived in England. The foot used by craftsmen supplanted the longer foot used in agriculture. The agricultural foot was reduced to 10⁄11 of its former size, causing the rod, pole or perch to become 16+1⁄2 (rather than the older 15) agricultural feet. The furlong and the acre, once it became a measure of the size of a piece of land rather than its value, remained relatively unchanged. In the last thousand years, three principal pounds were used in England. The troy pound (5760 grains) was used for precious metals, the apothecaries' pound, (also 5760 grains) was used by pharmacists and the avoirdupois pound (7000 grains) was used for general purposes. The apothecaries and troy pounds are divided into 12 ounces (of 480 grains) while the avoirdupois pound has 16 ounces (of 437.5 grains). The unit of volume, the gallon, has different values in the United States and in the United Kingdom, with the US gallon being 83.26742% of the imperial gallon: the US gallon is based on the wine gallon used in England prior to 1826. There was a US dry gallon, which was 96.8939% of an imperial gallon (and exactly 1+15121/92400 of a US gallon), but this is no longer used and is no longer listed in the relevant statute. After the United States Declaration of Independence the units of measurement in the United States developed into what is now known as customary units. The United Kingdom overhauled its system of measurement in 1826, when it introduced the imperial system of units. This resulted in the two countries having different gallons. Later in the century, efforts were made to align the definition of the pound and the yard in the two countries by using copies of the standards adopted by the British Parliament in 1855. However, these standards were of poor quality compared with those produced for the Convention of the Metre. In 1960, the two countries agreed to common definitions of the yard and the pound based on definitions of the metre and the kilogram. This change, which amounted to a few parts per million, had little effect in the United Kingdom, but resulted in the United States having two slightly different systems of linear measure, the international system and the surveyors system, until the latter was deprecated in 2023.
English units of measure
English units of measure were derived from a combination of Roman, Carolingian and Saxon units of measure. They were a precursor to both the imperial system of units (first defined in 1824, to take effect in 1826) and United States customary units which evolved from English Units from 1776 onwards. The earliest records of English units of measure involve the weight (and therefore the value) of Saxon coins. The penny introduced by Offa was about 20 grains (1.296 g). Edward the Elder increased the weight of the English penny to 26 grains (1.685 g), thereby aligning it with the penny of Charlemagne. By the time of the Norman Conquest (1066), it had decreased to 24 grains (1.555 g). This value was subsequently called the pennyweight and formed the basis of the Troy units of weight—the troy ounce used to this day for weighting precious metals. Edward I (1272–1307) broke the link between a coin's value and its weight when he debased the English coinage by introducing a groat (four pence) which weighed of 89 grains rather than the expected 96 grains. The groat was further devalued in the 1350s when its weight was reduced to 72 grains.
During Saxon times land was measured both in terms of its economic value and in terms of its size. The Domesday Book used the hide, an economic unit of measure. In other references the furlong and the rood appear to be units related to ploughing procedures. Of particular interest was the rood which was 15 North German feet in length, the North German foot being equivalent to 335 mm (13.2 inches). Craftsmen, on the other hand used a shorter Roman foot.
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