The pronominal adjective ὁπόστος has been disregarded by most reference grammars of Ancient Greek. This fact could be explained by its scarce attestation in the extant literature, presumably due to its high level of semantic specialization («in what or which place in numerical order»). Therefore, this paper aims to analyse all the evidence available in order, first, to identify the syntactic functions of ὁπόστος and, second, to try to trace the link between them. The analysis has revealed three uses of ὁπόστος, namely, as an indefinite, as a relative and as an indirect interrogative. Its morphology suggests that the use as an interrogative is the original, despite being the less attested. In effect, ὁπόστος follows the pattern of formation of indirect interrogative pronouns (sc. frozen ὁ plus a direct interrogative pronoun). The use as a relative could have emerged from contexts where more than one interpretation was structurally possible, mostly on dependence of utterance, perception and cognition verbs. Lastly, the use as an indefinite probably derives from generic relative clauses with elliptical verb.