English unit is the American name for a unit in one of a number of systems of units of measurement, some obsolete, and some still in use in present-day USA, the UK, and elsewhere. The modern United States customary units system as a whole is not, and has never been, used in England, and the modern Imperial units system as a whole is not, and has never been, used in the United States. Both of those more specific subsystems are part of the English units.
Most of the units are based on, but not necessarily the same as, old units used in historical England. In spite of the name, 'English unit' does not refer to the (non-SI) system of units with similar names still in widespread but unofficial use in England and the rest of the United Kingdom. These other units with similar names, but often different sizes, in use the UK are called the Imperial System, and in most but not all cases have been phased out in favour of the Metric System.
Various standards under the name 'English units' have applied at different times, in different places and for different things. Prior to the Battle of Hastings in 1066 the Anglo-Saxon system of measurement had been based on the units of the barleycorn and the gyrd (rod), inherited from tribes from Germany. After the Norman conquest, Roman units were reintroduced. The resultant system of English units was a combination of the Anglo-Saxon and Roman systems.
Later development of the English system continued by defining the units by law in the Magna Carta of 1215, and issuing measurement standards from the then capital Winchester. Standards were renewed in 1496, 1588 and 1758. The last Imperial Standard Yard in bronze was made in 1845; it served as the standard in the United Kingdom until the yard was internationally redefined as 0.9144 metre in 1959 (statutory implementation: Weights and Measures Act of 1963).
Usage of the term "English System" or "English Unit" is common in the US, but it is problematical. It can be ambiguous. It usually refers to either the Imperial System or the US Customary System, and in cases where these two systems differ, it is not clear which system is being described. Some people also call it the "British system" in the US. It is interesting to note that referring to this system as the British or English system almost only occurs in the United States, mainly causing confusion in the United Kingdom when reading from American sources.
four linear rods. Named after the length of surveyor's chain used to measure distances until quite recently. Any of several actual chains used for land surveying and divided in links. Gunter's chain, introduced in the 17th century, is 66 feet.
"One plough's furrow long" (Saxon furrow is furh), the distance a plough team could be driven without rest. This varied from region to region depending on soil type and local habit. In modern context, it is deemed to be 660 feet, 40 rods or ten chains.
Usually three miles. Intended to be an hour's walk.
Area
perch
one rod, when referring to length; one square rod when referring to area; one rod by one foot by a foot and a half when referring to volume (usually specifically for masonry stonework)
area of land one chain (four rods) in width by one furlong in length. As the traditional furlong could vary in length from country to country, so did the acre. In England an acre was 4,840 square yards, in Scotland 6,150 square yards and in Ireland 7,840 square yards. It is a Saxon unit, meaning field. Probably meant to be "as much area as could be plowed in one day".
one quarter of an acre, confusingly sometimes called an acre itself in many ancient contexts. One furlong in length by one rod in width, or 40 square rods.
four to eight bovates. A unit of yield, rather than area, it measured the amount of land able to support a single household for agricultural and taxation purposes.
or wapentake - 100 hides grouped for administrative purposes.
Volume
General
In both the United Kingdom and America, in addition to perch as a measure of length, there is also the perch which refers to the volume measurement of stone; one perch is equal to 16.5 ft × 1.5 ft × 1 ft = 24.75 cu. ft. of dry stone. The relationship to the unit of length (one perch = 16.5 feet) should be obvious.
Cup × 2 = 16 oz. (U.S.) or 20 oz (imperial) (and a "Pint's a pound the world around" or in the United Kingdom, "A pint of pure water weighs a pound and a quarter")
Chart showing the relationships of volume measures.
A Tun would actually be about 2,048 lb. but is a pretty close estimate, given that you would derive the weight and volume all from mouthfuls of water.
Wine
Brewery
Weight
Chart showing the relationships of weight measures.
The Avoirdupois, Troy and Apothecary systems of weights all shared the same finest unit, the grain, however they differ as to the number of grains there are in a dram, ounce and pound. Originally, this grain was the weight of a grain seed from the middle of an ear of barley. There also was a smaller wheat grain, said to be (barley) grains or about 48.6 milligrams.
The Troy and Tower pounds and their subdivisions were used for coins and precious metals. The Tower pound, which is based upon an earlier Anglo-Saxon pound, was abolished in 1527.
In terms of (silver) currency a pound was 20 shillings of 12 pennies each (i.e. 240) from the late 8th century (Charlemagne/Offa of Mercia) to 1971 in the United Kingdom, but lighter than a troy one. Most old European currencies, like mark, shilling/solidus/groschen/øre, penny/pfennig/denar, taler/dollar/krone, florin/gulden/guilder/pound/z?oty also belong into this monetary system.
The carat was once specified as four grains in the English-speaking world.
Some local units in the English dominion were (re-)defined in simple terms of English units, such as the Indian tola of 180 grains.
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