Mapping of postatrial fibrillation ablation atrial tachycardias (ATs) is often challenging because multiple foci may coexist and extensive atrial substrate has been targeted. It is often necessary to create multiple activation maps in the event of AT changing during mapping or in the case of multiple AT foci or mechanisms. To rationalize activation and entrainment mapping maneuvers, it is critical for the operator to have a probabilistic clue concerning the chamber of interest, constantly updated after each diagnostic step. This article describes the clues to distinguishing right from left ATs at each of 4 steps in the diagnostic process.