The prehensile tail may have evolved twice (in parallel) in New World monkeys (platyrrhines), suggesting it is an effective adaptation to negotiating arboreal habitats. Yet, despite the obvious importance of the prehensile tail for balance, feeding behavior, and locomotion, the structural differences between prehensile and nonprehensile tails are poorly understood. Previous studies showed that some linear measurements of caudal vertebrae are capable of distinguishing prehensile from nonprehensile tails but only in the distal parts of the vertebral sequence. This study examines structural properties of the tail with external measurements that are selected to better approximate resistance to bending/torsion while also examining vertebral cross-sectional geometry with computed tomography—a direct measure of resistance to bending/torsion. Specifically, this study tests the hypotheses that the caudal vertebrae (and the tail as a whole) of prehensile-tailed platyrrhines are structured to resist higher torsional and bending stresses than their functional analogues in nonprehensile-tailed platyrrhines, and that the predicted differences become more drastic further distally within the sequence. Results of this study indicate that prehensile and nonprehensile tails are structured differently. Prehensile tails are characterized by longer proximal tail regions than nonprehensile tails. Furthermore, the hemal processes (the distal attachment for the primary tail flexors) of prehensile tail vertebrae are better developed and can distinguish prehensile from nonprehensile tails better than traditionally used external measurements. Finally, results confirm predictions that prehensile tail caudal vertebrae are capable of withstanding higher torsional and bending stresses than their nonprehensile tail counterparts, and that these disparities become more pronounced further distally within the sequence. Anat Rec, 293:730–745, 2010. © 2010 Wiley-Liss, Inc.