An obsolete decimal multiplier prefix in the metric system, 1798 – 1935, indicating 10⁴, that is, 10,000, of the unit.

The myria’s symbol was one of the factors that led to its demise. In 1905 the CIPM assigned it the symbol M, so that myriameter, for example, was abbreviated Mm.¹ But in the first part of the twentieth century, electrical engineers began to use capital M for the prefix mega-, to mean one million, 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 a symbol for it, dropping the myria- entirely.² In addition, it had become clear that adopting prefixes for every power of ten was counterproductive. It is optimal to have names only for those exponents that are multiples of 3.

After “M” was dropped the abbreviation for the prefix myria- became “my-”.³

In 1975, the United States, having authorized use of the myriameter and myriagram in the Act of July 28, 1866, declared the terms no longer acceptable.⁴

1. J. René Benoît.
Annex to the Proces Verbaux.

2. Proces Verbaux, volume 17 (1935). Page 76.

3. See for example U.S.G.P.O. Style Manual (1967), page 163, which gives “mya., myriare”, “myg., myriagram”, “myl., myrialiter” and so on. Note the use of periods.

4. “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, millier or tonneau, quintal, myriagram, kilo (for kilogram)”

Guidelines having the force of law issued by the Director of the National Bureau of Standards, 1 June 1975. Published in Federal Register Doc.75-15798 Filed 6-18-75:8:45 am. See Appendix 7, page 33, of Lewis V. Judson, Weights and Measures of the United States, a Brief History. NBS Special Publication 447.  1976 (rev. of 1963 ed).


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