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A perch is as a unit of measurement used for length, area, and volume in a number of systems of measurement. Its name derives from the Ancient Roman unit, the pertica.
The word perch is from the French perche, derived from the Latin pertica, meaning a pole or staff.[1] Originating in Roman antiquity, it spread with the Roman Empire and was likely re-introduced to England with the Norman Conquest of 1066. In the Roman Empire, France and England, it also could mean area (square perches), and among operative masons of the Middle Ages, volume.
The perch as a lineal measure in Rome, was 10 feet (3.05 m), and in France varied from 10 feet (perche romanie) to 22 feet (perche d'arpent - apparently 1/10 of "the range of an arrow". - about 220 feet). To confuse matters further, by ancient Roman definition, an arpent equalled 120 Roman feet.
In England, the perch was officially discouraged in favour of the rod as early as the 15th century[2], however local customs maintained its use. In the 13th Century Perches were variously recorded in lengths of 18 feet (5.49 m), 20 feet (6.1 m), 22 feet (6.71 m) and 24 feet (7.32 m); and even as late as 1820, a House of Commons report notes lengths of 16.5 feet (5.03 m), 18 feet (5.49 m), 21 feet (6.4 m), 24 feet (7.32 m), and even 25 feet (7.62 m)[3]. In Ireland, a perch was standardized at 21 feet (6.4 m), making an Irish chain, furlong and mile proportionately longer by 27.27% than the "standard" English measure.[4]
The rod as a survey measure was standardized by Edmund Gunter in England in 1607 as one-fourth of a chain (of 66 feet / 20.12 metres), or 161⁄2 feet (5.50 yards; 5.03 metres) long.
As a unit of area, a square perch (the perch being standardized to equal 16.5 feet) is equal to a square rod, 301⁄4 square yards (25.29 square metres) or 0.00625 acres. There are 40 square perches to a rood, 160 square perches to an acre. This unit was usually referred to as a perch or pole even though square perch and square pole were the more precise terms. Confusingly, rod was used as a unit of area but it meant a rood.
The Perch remains a standard unit of land area used in Sri Lanka being equivalent to 25 square metres.
Obviously, regional interpretations of perch would yield different results.
A traditional unit of volume for stone and other masonry. A perch of masonry is the volume of a stone wall one perch (16.5 feet / 5.03 metres) long, 18 inches (45.7 cm) high, and 12 inches (30.5 cm) thick. This is equivalent to exactly 24.75 cubic feet (0.916667 cubic yards; 0.700842 cubic metres).