Consistency,as coherence, is a central requirement of justice, and, consequently, a central requirement for any legal system. The concrete conditions to achieve it, are, however, less evident. The potential reach of the concept as it appears in the case law of the ECJ, is still mostly unexplored in academic literature. This article thus undertakes to analyse and pinpoint its significance, taking direct tax law as an example of a subject matter falling squarely within the competence of the Member States. As such, direct taxation provides a prime example for the conflicting principles of national parliamentary sovereignty and quasi-constitutional supranational limits to this sovereignty and thus an ideal case for such analysis. It is shown how many of the peculiarities of the ECJ decisions on tax discrimination can bereconceptualized and understood in terms of the idea of consistency. Through the discussion of the main justifications unique to direct tax cases the article also reveals the inconsistency of the ECJ in respect of its application of that central idea and advocates a more stringent approach to improve the transparency of the case law and thus, in turn, its consistency. This approach is explained in more detail by reference to different tests for consistency to be employed on each stage of the proportionality analysis, and defended against possible objections to different standards of the concept.