Approximate measures are units of volumetric measurement which are not defined by a government or government-sanctioned organization, or which were previously defined and are now repealed, yet which remain in use.[1][2][3]
It may be that all English-unit derived capacity measurements are derived from one original approximate measurement: the mouthful, consisting of about 1⁄2 ounce, called the ro in ancient Egypt (their smallest recognized unit of capacity).[4][5] The mouthful was still a unit of liquid measure during Elizabethan times.[6] (The principal Egyptian standards from small to large were the ro, hin, hekat, and khar.)[7]
Because of the lack of official definitions, many of these units will not have a consistent value.
{{cite book}}
: CS1 maint: location missing publisher (link)
Who would have thought that the units of measure "the pint" and "the quart" are based on "the mouthful" (Klein, 1974, The World of Measurements: Masterpieces, Mysteries, and Muddles of Metrology.).
The smallest recognized unit of volume was the ro, a mouthful. It was reckoned that five mouthfuls made one sixty-fourth of a heqat so there were 320 ro to one heqat.
A fairly clear line of descent has thus been traced from the jigger, or handful, of Elizabethan England to the customary unit for dispensing the "firewater" that is the most prevalent drug used in our own time and culture, nearly four centuries later. In the United States the half jigger, sometimes called a pony, is half again the Elizabethan mouthful.