is generally accepted that concurrency can be difficult for students to reason about. While some studies provide insight into the nature of these difficulties [6], work remains to be done in understanding the aspects of learning about concurrency that are most difficult, and in developing approaches to dealing with this problem. We have conducted instructor interviews and an observational study of students, identified several key difficulties that students encounter, and developed a diagram that we believe will be an aid to understanding and problem-solving. We present the diagram and results of an initial user evaluation.