This is the first English language paper seriously to examine the meaning of subsidiarity from the perspective of private law, in which it might be used to understand legal rules, or the interaction of different kinds of claim. Since there are so few relevant sources in English, this article casts a wide net for consensus. It offers six propositions about what it means to designate a rule or relationship (between legal regimes, say) as one of subsidiarity. These are formulated by reference, principally, to thinking about subsidiarity outwith private law; and, secondarily, to (i) miscellaneous literature about subsidiarity, (ii) the general French private law literature about subsidiarity, and (iii) what little can be gleaned from relevant unjust enrichment discourse in English. The state of play in that discourse is summarised, before the choice of Roman Catholic social teaching, European Union law, and European human rights law as settings to examine for their conceptions of subsidiarity is explained, and subsidiarity in each of these contexts is sketched out. Succeeding sections then outline each proposition, and clarify how it may be derived from the sources. The paper concludes by reflecting guardedly on the potential of subsidiarity in private law, as a way to model the interrelation of private law claims and doctrines.