The cun (Chinese: 寸; pinyin: cùn; Wade–Giles: ts'un; Japanese: sun; Korean: chon) 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 all fingers side-by-side is three cuns. 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 called a "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).
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 and 5/8 of an inch. A metric ruler is shown next to it for comparison