Introduction Three important spaces of the shoulder that are bordered by the triceps include quadrangular space triangular space triangular interval Quadrangular Space Borders medial: long head of triceps lateral: humeral shaft superior: teres minor inferior: teres major Contents axillary nerve passes through the quadrilateral space on its path to innervate the teres minor and deltoid and provide sensation to the lateral arm posterior humeral circumflex artery Triangular Space Borders inferior: teres major lateral: long head of triceps superior: lower border of teres minor Contents scapular circumflex artery Triangular Interval Borders superior: teres major lateral: lateral head of the triceps or the humerus medial: long head of the triceps Contents profunda brachii artery radial nerve