A cun is not a fixed centimetre. It is a part of the body of the person you are studying. The same phrase — three cun — lands in a different place on each anatomy.
Two references from the first week
Thumb width at the joint is studied as 1 cun. Four fingers held together — index to little finger — are studied as 3 cun. They are classroom references for orientation, not a clinical measuring tool.
Two points where the cun becomes concrete
LI4 (Hegu) is studied on the back of the hand, toward the middle of the second metacarpal: the cun is short because the hand is short. ST36 is studied about 3 cun below the knee, one finger lateral to the tibial crest. The 3 of the hand and the 3 of the leg are not the same number of centimetres.
How to practice it
Open the sheet, read the location, then the 3D body, and only then take the proportion to your own hand or a classmate. A number without the neighbouring bone locates nothing.
LI4 sheetST36 sheetSP6 sheetLocation drill
Questions
- Can I use a centimetre ruler?
- Not if you are studying the idea. The cun changes with the person. A fixed ruler teaches a distance the next body does not have.
- The patient's cun or the practitioner's?
- Location study uses the body where the point sits. Your own hand-cun is only a teaching comparison, not a stencil on someone else.
Study reference within traditional Shiatsu/TCM frameworks. Does not diagnose or treat.