"Although the measurements in the metric system are derived from scientific principles, the English units measurements (and the subsequent USA and Imperial measurements) are based on nature and everyday activities.

"For example, a league is based on the distance that can be walked in an hour. Sailors (in days gone by) would drop a weighted rope into the water, lowering it by lengths (where each length was measured by holding the rope between their outstretched hands) until the weight at the end of the rope touched the seabed. This led to the definition of the fathom as the distance from the fingertips of one hand to the fingertips of the other, when the hands are held straight out to the sides. A grain (used to measure small quantities of precious metals) is the weight of a grain of wheat or barleycorn.

"Such natural measures were well suited in a simple agricultural society. However, as trade and commerce grew, it was necessary to have more consistent measures (after all, not all grains of wheat have the same weight and not all sailors have the same length of arm). Consequently, metal weights and lengths were produced to represent exact measures; these metal representations where then used to produce official scales and measurements to ensure that trade was based on standard quantities.

"For larger measures (e.g. a mile) it was impractical to build a metal equivalent, so they were redefined to be multiples of the smaller measures. It is for this reason that the mile was changed in 1595 under Queen Elizabeth I's reign from the Roman standard of 5,000 feet to 5,280 feet (which is 8 furlongs, each furlong equal to 10 chains, each chain equal to 22 yards and each yard equal to 3 feet).

"Despite the development and standardization of the English units of measure, their roots in ancient agriculture and trade have resulted in a diverse and relatively complex set of measurements. The various trades each developed their own measures, so in many cases the measure would depend on what it was being used for: a barrel of oil is not the same size as a barrel of wine (there are in fact eight different barrel sizes officially recognized).

"Likewise there are both fluid ounces and weight ounces, with different types of weight ounces (depending on what was being weighed). This complexity was not eliminated when the English system evolved into the Imperial and USA systems, with the result that these systems have approximately 300 different units of measurement. In comparison, the metric system has only 7 basic units of measurement (which can be increased or decreased in multiples of 10 to make larger or smaller units, or combined to make more complex units)."

From:http: //www.france-property-and-information.com/imperial-system-and-history.htm