Monme (A Weight Unit in The Japanese Traditional System of Weights And Measures) ()

Monme is a weight unit in the Japanese traditional system of weights and measures. The Weights and Measures Act, promulgated in 1891, set the weight of 1 monme at 3.75 grams.

It was only in the Meiji period that the name monme officially began to be used as the weight unit, and until then, "sen" had been used instead. As the name itself suggests, sen came from the weight of 1 mon-sen ('mon' was a former Japanese currency unit, and 'sen' means 'coin').

In China in and after Northern Sung Dynasty, a coin was minted in a manner that the weight of 10 coins of Tang dynasty called Kai Yuan Tong Bao (or Kai Tong Yuan Bao) equaled 24 zhu, that is to say, 1 ryo (tael), and a weight of the one coin was called 'sen' (銭). Therefore, a tenth ryo was 1 sen. However, mass-produced metallic currency was difficult to be equalized in weight (at worst, the disparity was as large as almost twice) so Kai Yuan Tong Bao could not be said the standard of the weight of currencies.

When this unit sen was introduced into Japan, it was called 'monme,' which was an abbreviation for mekata (weight) of 1 mon-sen. The Chinese character '匁' (monme) is said to have originated from the cursive style writing as '泉,' a homonymy of 銭, or from the Chinese character '文' (mon) attached 'メ' (me), a character of katakana (the square form of kana [Japanese syllabaries]). In the Edo period, the currency value of Hyoryo ginka (the silver coin used as the currency by weight) depended on its weight, and in 1700, 60 monme of chogin (silver coin) was officially equalized in value with 1 ryo of koban (former Japanese gold coin of oval shape), however, it was actually floating rate by the market economy.

In 1665, the Japanese weight units were standardized, and thereafter, Shirobei GOTO and his descendants monopolized the right to produce counterweights used at an exchange house, and the production and use of other kinds of counterweights were prohibited strictly for the prevention of illegal exchange. For these counterweights, 'ryo' was used as a base unit and 'monme' a secondly, while 'monme' and 'kan' were used for the currency unit for Hyoryo ginka, to avoid being mixed up with 'ryo,' the currency unit for koban. That is, chogin balanced with 5 ryo of the counterweights was said as 50 monme of silver. This made 'monme' spread as a weight unit.

At that time, 1 monme was set between 3.74 to 3.75 grams, slightly lighter than 3.75.

In the Meiji period, sen was ranked the secondary currency unit of a hundredth value of 1 yen in Japan. Instead, monme was officially adopted as the weight unit by the New Currency Act in 1871 (though sen was used in parallel before World War II) and 1 monme was equalized with a thousandth kan, and the weight of 1 monme was set at 3.756521 grams. After that, for convenience in conversion among units, the Weights and Measures Act set 1 kan at 3.75 kilograms, so 1 monme became equal to 3.75 grams.

And in China, whose weight units influenced Japanese counterparts through commerce from ancient times, 1 sen was set at 4.1762 grams in Sui dynasty, and at 3.7301 grams in the Tang dynasty, says "History of Chinese Weights and Measures" written by Wu Chengluo.

1000 times 1 monme is 1 kan, and 10 times 1 monme is 1 ryo, and a tenth of 1 monme is 1 fun, and a hundredth of 1 monme is 1 rin, and a thousandth of 1 monme is 1 mo. Accurately, however, fun, rin, and mo are the units to show the proportion of decimal fractions to 1. And here, it is called 'fun' rather than 'bu' to avoid mixing up with bu which was used as the currency unit of a gold coin, ichibu-kin (a former Japanese gold coin of 1 bu).

Originally, for decimals in Chinese numeral, a tenth is represented as bu, a hundredth as ri, and a thousandth as mo, while a rate to represent a tenth is 'wari' in Japan which is the base of the rate unit, and a hundredth becomes bu and a thousandth ri.
This was because the basic weight unit in Japan was 'ryo.'

Monme, along with kan, is still used as an international weight unit for a pearl. This is because a pearl was a Japanese specialty. For a pearl, monme is written as momme and symbolized as mom. The Japanese Measurement Act has abolished almost all units used under the traditional system of weights and measures, but it allows monme (written in 'もんめ' in hiragana [the cursive form of kana, the Japanese syllabary]) to be used merely for weighing a pearl.

The Japanese 5 yen coin has the weight of 3.75 grams, or 1 monme.

Incidentally, when the unit's digit is zero, monme (匁) is occasionally replaced with 'me' (目). For example, 30 monme can be replaced with 30 me, and 300 monme with 300 me. However, the me used as '10 me,' '100 me' and others is not a weight unit, so, for example, 232 monme cannot be written as 200 me plus 32 monme. When the unit's digit is not zero, weight should always be written with monme. Therefore, 17 monme cannot be written as 17 me.

In China, 'sen' serves as the weight unit even today, and after the metric system was adopted, 1 sen was set at 5 grams. 10 times 1 sen is 1 ryo, and a tenth of 1 sen is 1 bu, which are both the same as Japan.
(Also refer to the article of Chinese Traditional System of Measurement)

[Original Japanese]