Relative terms are those such as ‘larger’, ‘smaller’, ‘parent’ and ‘offspring’. Questions concerning the nature of this type of term in Plato fall under three themes. First, logic: what is the syntax and semantics of relative terms? Second, metaphysics: what structures in the world constitute relative properties? Third, taxonomy: do relative terms form a distinguishable class? Questions concerning purpose ask what role these terms have in the wider economy of Plato’s thought. Only one existing approach addresses all of these themes and questions: it was put forward by G.E.L. Owen in 1957, although it was subsequently developed by others. The Owenian view holds that relatives are syntactically or semantically incomplete, that they are identical to metaphysically dyadic relations and that they do form a taxonomic class. According to Owen, Plato introduces relative terms to bolster a certain argument for the separation of forms and participants. Therefore, they have an ontological purpose. This thesis aims to offer a plausible, non–anachronistic alternative to the Owenian view. To give such an account I have to argue for a radically different logic, metaphysics and purpose for relatives in Plato. I call the view that I defend ‘conjunctivism’. I begin by characterising the logic of conjunctivism. Plato holds that relative terms have formal objects. These are exceptionlessly correct objects of the relative in question. A parent is always and only parent of offspring, so ‘offspring’ is the formal object of ‘parent’. I then demonstrate that the metaphysical problems for relatives which are not dyadic relations are avoided by Plato’s version of conjunctivism. Looking at Sophist 255c–d and Parmenides 133c–134e, I discuss the taxonomy of relative terms. I show that, under the conjunctive reading, they form a distinguishable class and, in contrast to Owenian relatives, each reciprocates with its correlative. So, just as a parent is relative to offspring, so offspring are relative to a parent. With the nature of relative terms established, I proceed to refute Owen’s account of their purpose, and give my own explanation. By looking at passages from the Euthydemus and Charmides, I argue that Plato introduced relative terms to articulate why some arguments are fallacies and others not. That is, relative terms have a dialectical purpose.