The posterior divisions from each of the three trunks of the brachial plexus join behind the clavicle to form the posterior cord. Three terminal branches arise directly from the cord at this level: the upper subscapular nerve, thoracodorsal nerve, and lower subscapular nerve. The main trunk continues distally, terminating as the axillary and radial nerves. The axillary nerve innervates the deltoid and provides a sensory branch to the skin overlying this muscle. The radial nerve continues into the arm and divides into superficial and deep branches at the elbow, the former supplying sensation to the dorsal hand, and the latter providing innervation to muscles in the extensor compartment of the distal arm. Variations at every level are described, extending from the nerve roots contributing to the plexus, to the pattern of innervation over the dorsal hand.