This paper posits a distinction between plain definite DPs and indexed definite DPs: only the latter have a syntactically represented referential index, an idxP, in [Spec, DP]. After sketching a theory of idx as a cross-categorial feature of pro-forms, the paper argues that anaphoric definites, complex demonstratives (e.g. that linguist), and pronominal definites (e.g. we linguists) have idxP specifiers in [Spec, DP]. The second part of the paper presents a case study from anaphoric definite DPs in Marka-Dafing (Mande) which provides evidence for this proposal. We examine a co-occurrence restriction between exophoric and anaphoric demonstratives in Marka-Dafing, concluding that this restriction provides additional support for a syntactically represented idxP in [Spec, DP], as well as motivating the Single Index Constraint, the idea that indexed definites can only make use of a single index to achieve reference.