Myria- (symbol my) is a now obsolete decimal metric prefix denoting a factor of 104 (ten thousand).[1][2][3][4][5][6] It originates from the Greek μύριοι (mýrioi) (myriad). The prefix was part of the original metric system adopted by France in 1795,[7] but was not adopted when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
In 1685 John Wallis proposed the usage of myrio.[8] Also, in 19th century English it was sometimes spelled myrio,[9][10] in line with a puristic opinion by Thomas Young.[2][3][11]
The myriametre (10 km) is occasionally encountered in 19th-century train tariffs, or in some classifications of wavelengths as the adjective myriametric. The French mesures usuelles (1812-1839) did not include any units of length greater than the toise, but the myriametre remained in use throughout this period.[12] In Sweden and Norway, the myriametre is still common in everyday use. In these countries this unit is called mil. Of units customarily used in trade in France, the myriagramme (10 kg) was the metric replacement for an avoirdupois unit, the quartier (25 pounds). Isaac Asimov's novel Foundation and Empire still mentioned the myriaton in 1952.
The myria's symbol of my ultimately led to its demise. In 1905 the Comité International des Poids et Mesures (CIPM) assigned it the symbol M, wishing to use only single-letter symbols. This meant that myriameter, for example, was abbreviated Mm.[13] But in the first part of the twentieth century, electrical engineers began to use capital M for the prefix mega-, as in megawatt and megohm. This usage became so widely and firmly adopted that in 1935 the CIPM adopted the prefix “mega-” with “M” as its symbol, dropping the myria- entirely.[14] In 1975, the United States, having previously authorized use of the myriameter and myriagram in 1866, declared the terms no longer acceptable.[15][16]
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Prefixes for Naming Multiples and Submultiples of Units: […] The prefix "myria" is sometimes used for 104 and "lakh" for 105.
And yet if this Number be not thought large enough, let all these (saith he) be called the First Period; and this Unit, a Unit of the first Numbers of the second Period; and (after so many places more) a Unit of the first Numbers (or Classis) of the third Period; and so onward to a Myriad of Myriads of such Periods: The last Unit of which, that is, one Myriad of Myriads, of the Myrio-myresimal Classes, of such Myrio-myresimal Period, answers (in our Notation) to 1 followed by 8, 0000, 0000, 0000, 0000, Ciphers: That is, 1 with Eight Myriads of Myriads of Myriads of Myriads of Ciphers.
10 kiloliters = 1 myrioliter
The myriogram equals 22.046 lbs. avoirdupois.
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Accordingly, the following units and terms listed in the table of metric units in section 2 of the act of July 28, 1866, that legalized the metric system of weights and measures in the United States, are no longer accepted for use in the United States: myriameter, stere, millierortonneau, quintal, myriagram, kilo (for kilogram).
