Concurrent software is notoriously difficult to debug. We investigate the use of UML sequence diagrams to help developers correctly reason about the potential behaviors of buggy concurrent software. We conducted a controlled experiment that compared internal (i.e., "in the head") and external representations for reasoning about multithreaded software. For external representations, participants created multithreaded sequence diagrams. The results of the experiment demonstrate a strong positive effect associated with using external representations. Participants who drew diagrams were significantly more successful at reasoning about the potential behavior of concurrent software. Moreover, participants who produced diagrams with higher levels of detail and with fewer errors tended to achieve greater levels of success. Additionally, this paper contributes an extension to the UML sequence diagram notation for showing behavior of multithreaded software and formal metrics for assessing the complexity of thread interactions.