On the midaxillary line, 3 cun below the apex of the axilla.
How to locate
Abduct the arm slightly and find the axilla apex. Measure 3 cun inferior to it in the 4th intercostal space.
The 4th intercostal space is in level with the nipples, which is more obvious to identify for men. For women, it is more reliable to use manubriosternal synchondrosis as reference.