Cun (length)

Cun
Wooden ruler of the western Han dynasty, unearthed at Jinguan Pass Site in Jinta County
Unit system	Chinese unit
Unit of	length
1 cun in ...	... is equal to ...
metric (SI) units	1/30 m  ~33.33 mm
imperial/US units	~0.10936 ft  ~1.3123 in

Tsun
A section of an old Hong Kong ruler, showing the last (10th) cun of a chi. One can see that the chi in that jurisdiction was exactly equal to 14 5/8 of an inch. A metric ruler is shown next to it for comparison
Unit system	Chinese unit
Unit of	length
1 tsun in ...	... is equal to ...
metric (SI) units	6998371475000000000♠0.0371475 m  ~37.15 mm
imperial/US units	0.121875 ft  1 37/80 in

The cun (Chinese: 寸; pinyin: cùn; Wade–Giles: ts'un; Japanese: sun; Korean: chon), often glossed as the Chinese inch, is a traditional Chinese unit of length. Its traditional measure is the width of a person's thumb at the knuckle, whereas the width of the two forefingers denotes 1.5 cun and the width of four fingers (except the thumb) side-by-side is three cuns.(http://www.tcmstudent.com/study_tools/Cun%20Measurements.html) In this sense it continues to be used to chart acupuncture points on the human body in various uses of traditional Chinese medicine.

The cun was part of a larger system, and represented one-tenth of a chi ("Chinese foot"). In time the lengths were standardized, although to different values in different jurisdictions. (See chi (unit) for details.)

In Hong Kong, using the traditional standard, it measures ~3.715 cm (~1.463 in) and is written "tsun". In the twentieth century in the Republic of China, the lengths were standardized to fit with the metric system, and in current usage in People's Republic of China and Taiwan it measures  3 ¹⁄₃ cm (~1.312 in).

In Japan, the corresponding unit, sun (寸（すん）, sun^?), was standardized at  ¹⁰⁰⁰⁄₃₃ mm (~3.030 cm, ~1.193 in, or ~0.09942 ft).