Understanding the reasoning of others is a key aspect to achieving a shared understanding when collaboratively solving a problem, such as the generation of a plan, and recent observations of military planners suggest that it plays a key role in the planning process. An example of rationale is described where a misunderstanding is only resolved by the joint exploration and cross-challenging of the rationale. A prototype tool is described that permits the creation and visualization of the basic rationale via the use of Controlled English (CE). Using the example, the paper explores mechanisms that could potentially make more effective use of rationale.