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The quadrangular (or quadrilateral) space is so named because of the shape of its anatomic boundaries (see Image. Quadrangular Space, Posterior View). The posterior humeral circumflex artery (PHCA) and axillary nerve exit the axilla and transit this space to their destinations within the posterolateral shoulder. The axillary nerve arises from the posterior brachial plexus cord and divides into musculocutaneous branches, typically before entering or within the quadrangular space.[1] The PHCA bifurcates into anterior and posterior branches inside the region.[2] The quadrangular space is a significant anatomic landmark for various diagnostic and surgical procedures. Quadrangular space syndrome (QSS) is a condition manifesting with axillary nerve or PHCA compression symptoms arising from various etiologies. Understanding this space's anatomy and functional significance is imperative for treating various upper limb conditions.