Sun (A Length Unit in The East Asian System of Weights And Measures) (寸)

Sun is a length unit in the traditional East Asian system of weights and measures. 1 sun is set at a tenth length of 1 shaku (unit of distance approximately equal to 30.3 centimeters). A tenth of 1 sun is 1 bu.
In the Heian period, sun was occasionally written as 'su.'
Sun (寸) is also read as 'ki' in a Japanese way of reading of Chinese character.

Definition
In Japan, during the Meiji period, 1 shaku was set at 10 over 33 meters, so 1 sun became 1 over 33 meters, or about 3.03 centimeters. This is the length measured with kane-jaku (a carpenter's iron square), and besides, there exists another length of sun measured with kujira-jaku (a stick used in kimono-making). 1 sun measured with kujira-jaku is about 3.8 centimeters.

In People's Republic of China, 1 shaku (in pinyin, 'chi') was set at 1 over 3 meters, so 1 sun (in pinyin, 'cun') became about 3.33 centimeters.
The Chinese character '寸' is also used for the decimeter (dm) of International Unit System (SI), so, to make a distinction, the sun used in their traditional system of measurement is called 'shi cun,' and the sun in SI is called 'gong cun.'

History
Sun is thought to have originated from an anthropomorphic unit for showing the breadth of a thumb (just the same as inch). Shaku originated from the breadth between the thumb and forefinger when they are opened, and sun seems to have been invented independently from shaku, but anyway, sun began to be set at the length of a tenth shaku during the age of Zhou Dynasty China. During the age of Han Dynasty China, the breadth of a grain of proso millet was set at 1 bu (in pinyin, 'fen'), and 10 bu was equalized with 1 sun. Varied as the length of shaku from time to time, the ratio of 1 shaku to 10 sun was kept.

The original shape of the Chinese character '寸' is the hieroglyphic of a hand with a horizontal line in its left. This represents the shape that a thumb is counting somebody's pulse by feeling the wrist, which is said to lead to the character's present meaning of the breadth of a thumb. Some says the description in "Setsumon" (Analytical Dictionary of Chinese Characters) that 'Go a bit (in Chinese characters, 一寸) from the lower end of the palm, and you will find sunko (寸口, the place where the pulse can be felt)' led the character '寸' to indicate the length.

Usage
In Japan, most adult men had the height from 5-shaku level (about 150 to 180 centimeters) so when their height was explained, '5 shaku' was omitted from their real height, and merely the remainder was mentioned in sun. For example, when someone said '4 sun for height,' it meant 5 shaku and 4 sun, and this was almost common sense when the East Asian traditional system of weights and measures was widely used. And when an incline was explained, sun served as the unit of vertical length per 1 shaku of horizontal length.

As the Japanese word 'sunpo' (length) shows, sun is used to express 'length;' and it also means short, small, or the like, which is seen in some Japanese words, such as 'sungeki' (a short play), 'sunshi' (literally, a small token of one's gratitude, which is used as a humble expression by a gift sender), 'sunpyo' (a brief comment), 'sundan' (cutting something into shreds), and 'chotto' (in Chinese characters, 一寸; for a while, or a little).