2.3 Indirect horizontal effect

20. The set of rules of Union law that currently is awarded direct horizontal effect is limited. Rules of Union law that lack direct horizontal effect may however still impact horizontal legal relationships. The Court of Justice has introduced various ways in which rules of Union law may indirectly influence horizontal legal relationships. From a private law perspective, this indirect influence of Union law on horizontal legal relationships is referred to as indirect horizontal effect.1 The term indirect horizontal effect applies to the following distinctive situations: 1) the mandatory consistent interpretation (or: harmonious interpretation) of law of national origin, 2) the compatibility review of a legislative provision against Union law2, and 3) positive obligations of Member States3.4 For the sake of completeness, linked to the obligation of consistent interpretation a fourth situation of horizontal indirect effect is the so-called voluntary consistent interpretation, which means that national courts interpret national law in conformity with Union law even when they are not obliged to.5 Exclusively the first two forms of indirect horizontal effect are relevant to the present study. In the following sections they are briefly discussed by way of introduction: first, the consistent interpretation of national law and, second, the compatibility review of provisions of national law against Union law. In the case studies these ways of indirect horizontal effect are discussed and illustrated comprehensively.