The base of each posterior triangle is the middle one-third of the clavicle. The medial margin is the posterior border of the sternocleidomastoid muscle, and the lateral margin is the anterior border of the trapezius muscle. The apex points superiorly and is immediately posteroinferior to the mastoid process.
The posterior triangles are associated with nerves and vessels that pass into and out of the upper limbs.
How to locate the cricothyroid ligament