Mastoid Crease

Behind the ear, near where the ear attaches to the scalp at the top, there is a valley in the heavy bone there, running usually from the ear toward the back of the head. It is called the mastoid crease. It can also be found just below the level of T4 and T6, partway between them.

On the left, the site is referred to as MC1, and on the right, MC2. Although A1 and A2 technically refer to ear lobes, the terms are commonly used interchangeably with MC1 and MC2.

A1 and A2 are inert sites, down below the level of brain. The mastoid crease is not and does carry a small amount of electrical signal.