Tan (a unit of area in old Japanese system of weights and measures) (反)
Tan (反 or 段) is a unit of area belongs to old Japanese system of weights and measures. There are two kinds of tan, one is to measure areas of lands and the other is to represent size of textile. There is another kind of tan represents the length of 6 ken (measure of length, 1 ken is 1.818 meters; 6 ken is 10.908 meters).
As an unit of land area
Tan as a unit of land area is a unit of multiples for tsubo (unit of land measurement; 1 tsubo is 3.31 square meters) which is the standard unit of area, and is now defined as 10 se (unit of area, 1 se is equal to 30 tsubo, about 99.174 square meters; 300 bu [歩; unit of land measurement; 1 bu is equal to 3.31 square meters]; 991.74 square meters). 10 tan is equal to 1 cho (unit of measurement; 109.09 m; 0.99 hectares).
In the Meiji period, 1 shaku (unit of length) was set to be 10 over 33 meters, therefore 1 tan now is equal to about 991.74 square meters, about 9.9174 ares. The unit is really close to 10 ares, thus the transition to the metric system of measurement was done smoothly.
In ancient time, an area of rice fields that brings 1 koku of rice was refered to as 1 tan. For that reason, although an area of 1 tan differs depending on the land's condition, lands with same numbers of tan can produce similar amount of crops. 1 koku of rice corresponded to the yearly consumption of rice per adult; therefore it was natural to make an area, brought 1 koku of rice, into a unit. The area was equal to approximately 360 bu; therefore 1 tan was set to be 360 bu (1,191.6 square meters). 1 tan has been 300 bu (993 square meters) ever since it was changed by Taiko-kenchi (the land survey by Hideyoshi TOYOTOMI). Some people say that hideyoshi set 1 tan into 300 bu to increase land tax, but some argue that he did because they could produce 1 koku of rice within an area of 300 bu by increasing of the production efficiency.
The rice yield per 1 tan of a rice field is called tanshu (production per tan). Today, under the metric system, the rice yield per 10 ares is called tanshu as well.
As an unit for the size of textile
The tan (反; also written as 端 in Chinese characters), representing a unit to measure textile, basically is equal to a cloth's width and length. The name 'Tanmono' (piece goods) comes from this fact.
This tan (反) comes from the tan (端), which is an unit of length used in ancient China. Although it has not been used since the Zhou period, a tan (端) represents 2 jo (unit of length, 1 jo is equal to 3.03 meters) that is 20 shaku (unit of length, approximately equal to 30.3 centimeters). The tan (端) was mainly used to measure fuhaku (cotton, silk, textile fabrics) in China and the Chinese character 反 (tan) was also applied after it has been imported into Japan.
The size that tan represents differ depending on times and materials of textile. In ancient time, 1 tan was equivalent to 9 sun (unit of length) and 5 bu (分; unit of length; approximately 28.785 centimeters) to 1 shaku (approximately 30.3 centimeters) in width and 2 jo and 8 shaku (approximately 8.48 meters) to 3 jo (approximately 9.09 meters) in length for silk clothes, and 9 sun and 5 bu (approximately 28.785 centimeters) in width and 2 jo and 8 shaku (approximately 8.48 meters) in length for menpu (cotton cloth; cotton material) (measured using kujira-jaku [unit of length, 1 kujira-jaku is approximately 37.9 centimeters]). Later, 1 tan as width and length of a cloth was set as 9 sun and 5 bu in width and at least 3 jo in length for a kimono (Japanese traditional clothing); 9 sun and 5 bu in width and at least 2 jo and 4 shaku in length for a menpu used for haori (Japanese half-coat); 9 sun and 5 bu in width and at least 2 jo in length for other clothes.