This paper investigates the Old Japanese (OJ) postposition -tu, a problematic morpheme with both genitive-like and numeral usages, and proposes a diachronic account of its distribution that eschews the traditional ‘locative-genitive’ analysis in favor of reconstructing pre-OJ *tu as a dual ‘allative / instrumental’ postposition. I show that both genitive and attributive uses of OJ -tu can be seen as developments out of a pre-OJ allative or directional sense, and show that reconstructing a pre-OJ allative can account for why OJ -tu is mostly found in locative expressions. I also propose that the use of OJ -tu in counting could reflect an original instrumental sense developing via a distributive, functions notably lacking among pre-OJ postpositions. Not only does this resolve syntactic problems in synchronic genitive analyses attempting to unify these two usages, it also explains why both bare numeral expressions and numerals suffixed with -tu are attested in Old Japanese. Reconstructing pre-OJ *tu as a dual ‘allative / instrumental’ reveals a striking match with the Middle Korean allative / instrumental particle -lwo, from which I reconstruct proto-Korean-Japanese *two via well-attested sound correspondences between Japanese and Korean. Unlike many of the synchronic analyses proposed thus far, this diachronic look at OJ -tu offers a unified analysis of its attestations that accounts for its locative distribution and even reveals a cognate with Korean. At a time when synchronic approaches predominate in our fields, this analysis helps highlight the value of diachronic approaches towards understanding distributions.