The usefulness of technical analysis, or charting, has been questioned because it flies in the face of the ‘random walk’ and tests present conflicting results. We examine chartists’ decision-making techniques and derive a taxonomy of charting strategies based on investors’ market ontologies and calculative strategies. This distinguishes between trend-seekers and pattern-seekers, and trading as a system or an art. We argue that interpretative activity plays a more important role than previously thought and suggest that charting’s main appeal for users lies in its power as a heuristic device regardless of its effectiveness at generating returns.