Measuring in cun

A cun is not a fixed length. It is a fraction of the body being measured, so a location written as “3 cun below the knee” lands further down a tall person’s leg than a short person’s. The WHO Standard Acupuncture Point Locations in the Western Pacific Region, published in 2008 by the WHO Regional Office for the Western Pacific, writes the locations of 361 points in this unit. Its foreword gives the reason: people differ too much in size for any absolute measure to work, and a WHO conference in Seoul adopted proportional measurement as the standard unit for point location in 1987. This page sets out the proportions the standard uses, the finger measures it allows as a check, and what studies found when they compared the two.

Three ways to find a point

The WHO standard names three methods and says they are often combined. The first uses anatomical landmarks, which it divides into fixed ones, such as bony prominences, the nipples, the navel and the nails, and movable ones that appear only when a joint or muscle moves. Its examples are GB34 (Yanglingquan), in the depression anterior and distal to the head of the fibula, and SI19 (Tinggong), in the depression in front of the tragus of the ear when the mouth is opened slightly. The standard also lists landmarks for counting down the spine: the spinous process of the 7th cervical vertebra is the most prominent one at the base of the neck, the 7th thoracic sits level with the lower tips of the shoulder blades, and the 4th lumbar sits on the line joining the tops of the hip bones.

The second is proportional bone measurement, set out in the next section. The third is finger cun, which measures the patient’s own fingers. The standard calls the first two the primary methods and keeps finger cun for points that are hard to locate with them. Where a classical text is unclear, one of its four rules for settling a location is that the anatomical landmark wins over finger-cun measurement when the two disagree.

Proportional bone measurement

Proportional measurement, gu du fa (骨度法, “bone measure method”), divides the distance between two landmarks into a set number of equal parts and calls each part one cun; ten cun make one chi. The WHO standard abbreviates it as B-cun and traces it to the chapter on bone measures (Gudu) in the Lingshu, one of the texts described in The classical texts, combined with measures added by later writers. The WHO International Standard Terminologies on Traditional Medicine of 2007 defines the same unit as body cun (同身寸): a length found by dividing part of the patient’s own body into equal divisions. The standard’s table of the main measurements is below, with the classical text it gives for each.

A note in the standard splits the 15 cun on the inner leg: from the lower border of the inner condyle of the tibia, where SP09 (Yinlingquan) sits, to the inner ankle bone is 13 cun, and the 2 cun above it are counted to the apex of the kneecap. The forearm shows how the table is read. The WHO standard puts PC06 (Neiguan) 2 B-cun above the palmar wrist crease, and the elbow-to-wrist span is 12 cun, so the point sits one sixth of the way up the forearm whatever the length of the arm.

China’s national standard for the same points, GB/T 12346-2021, Nomenclature and location of meridian points, keeps the counts and changes some of the ends. Its 2021 revision moved the end of the chest and upper abdomen spans from the xiphisternal junction to the tip of the xiphoid process, with the 9 and 8 cun unchanged, and added 3 cun from the posterior hairline down to below the spinous process of the 7th cervical vertebra. Where a person’s hairline is unclear, it counts 18 cun from the glabella over the head to below the 7th cervical vertebra. Its table also lists two spans the WHO table does not: 12 cun between the coracoid processes of the shoulder blades, and 2 cun from the base to the apex of the kneecap. Its foreword says the 2021 revision aligned its anatomical wording with the 2009 reprint of the WHO standard.

Finger cun

Finger cun, shouzhi tongshen cun (手指同身寸), measures the patient’s own hand and is abbreviated F-cun. The WHO standard defines three:

• Middle-finger cun: with the thumb and middle finger flexed into a ring, the distance between the ends of the two creases on the radial side of the middle finger’s middle joint is 1 cun.
• Thumb cun: the width of the thumb at its interphalangeal joint is 1 cun.
• Four-finger measure, called yifu fa (一夫法) in the Chinese standard: the index, middle, ring and little fingers held together, measured across the crease of the middle finger’s first finger joint, are 3 cun.

The WHO standard adds a fourth, the fingerbreadth: the width of the end segment of the middle finger, which it says should not be confused with the middle-finger cun and which it uses rarely, for ST06 (Jiache) and ST40 (Fenglong). The two standards give finger cun different reach. The WHO text says it is used mainly on the lower limbs, alongside proportional measurement, to verify a location. GB/T 12346-2021 says it is used mainly on the limbs, the face and the back, always on the basis of proportional measurement, and only for the few points the other two methods cannot fix.

What the studies measured

Studies that put the two units side by side have found that they disagree. A 2005 study by Yin and colleagues measured contemporary Korean patients and found finger-cun lengths differed significantly from proportional ones, varied with which finger measure was chosen as the standard, and differed further on the limbs of obese patients; the authors judged finger cun unreliable. A 2006 study by Park and colleagues measured bone lengths in 47 Korean women by dual-energy X-ray absorptiometry, a low-dose X-ray scan, and found one cun by the four-finger measure was significantly different from one cun by proportion.

Other studies measured how closely people land on the same spot. In a 2002 study by Aird and colleagues in Sydney, 72 acupuncture students repeatedly located a made-up point by four methods. Two newer methods, using a ruler and an elastic band, were both significantly more precise than the finger and proportional methods. For 95 in 100 attempts to land on the point, it would have needed to cover almost 13 cm² by the finger method and less than 3 cm² by the ruler. A 2012 study by Molsberger and colleagues asked 71 doctors, each with at least 200 hours of acupuncture training and two years of practice, to mark 23 points on the same man. The area holding 95 in 100 of their marks ran from 2.7 cm² for PC06 to 41.4 cm² for ST38 (Tiaokou); for ST36 (Zusanli) it was 20.7 cm². Points near clear landmarks at the forearm, ankle and back of the knee scattered least, and experience made no difference.

A 2019 systematic review by Debra Godson and Jonathan Wardle screened 771 studies and included 14, five of them on the cun system itself. Every study that tested finger cun found it significantly inaccurate or imprecise. The review found no study of how far qualified acupuncturists without a medical degree vary, and named the suitability of the other methods as a question still open.

How AcuiQ writes a location

AcuiQ’s point pages carry a written location from the catalogue, alongside a diagram. The written locations give distances in “thumb widths” where the WHO standard writes B-cun. The ST36 page reads “3 thumb widths below ST35”, and the WHO standard reads “3 B-cun inferior to ST35”; the PC06 page reads 2 thumb widths above the wrist crease, and the WHO standard 2 B-cun. The counts match the proportional ones in the WHO standard, so a thumb width on these pages stands for one proportional cun: 2 of the 12 on the forearm, in the case of PC06. On a person whose thumb is out of proportion to their forearm, a literal thumb gives a different spot, and the studies above found such differences are common. A few locations also use the middle-finger breadth, as the ST36 page does for the distance from the shin bone.

What the unit cannot settle

The WHO standard records its own unsettled cases. Its foreword reports that member states disagreed over about a quarter of the regular point locations; the consultations reviewed 92 disputed points, agreed 86, and left six with alternative locations for further research: LI19, LI20, PC08, PC09, GB30 and GV26. Even where the text is agreed, the Molsberger study found trained doctors marking areas of several square centimetres around a single point, and its authors proposed calling them acupuncture fields. They recommended that sham-controlled trials keep sham points at least 6 cm from real ones on the face, hands and feet and up to 12 cm elsewhere. Sham acupuncture and Reading an acupuncture trial take up what that means for a trial. How the points are named and numbered is in Reading a point code.