The posterior (lateral) triangle is a triangular-shaped area found in the posterior aspect of the neck. This area is also referred to as the lateral triangle, as this triangle is positioned more on the lateral than the posterior neck side. The following structures form its boundaries:

Apex - superior nuchal line of the occipital bone and union of the sternocleidomastoid muscle and the trapezius muscle;
Anterior - posterior border of the sternocleidomastoid muscle;
Posterior - anterior border of the trapezius;
Base - middle one-third of the clavicle.

The posterior triangle can further be subdivided into two smaller triangles by the inferior belly of the omohyoid muscle - occipital triangle and subclavian triangle.

Occipital triangle

The occipital triangle forms the superior aspect of the posterior triangle. Its apex, anterior border and posterior border match the same borders of the posterior neck triangle, but the inferior border is formed by the posterior margin of the inferior belly of the omohyoid muscle.

The floor of the occipital triangle is composed of the levator scapulae muscle, splenius capitis muscle and anterior scalene muscle, middle scalene muscle and posterior scalene muscle. The semispinalis capitis muscle sometimes also contributes to the formation of the floor of this triangle.

Subclavian triangle

The subclavian triangle is also known as the supraclavicular or omoclavicular triangle. It is smaller than the occipital triangle and is found below the inferior belly of the omohyoid muscle.

Its superior border is formed by the anterior aspect of the inferior belly of the omohyoid, its base or inferior border corresponds to the middle two-thirds of the clavicle, while its anterior border is formed by the sternocleidomastoid muscle. This triangle corresponds to the supraclavicular fossa.

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Posterior triangle