Italic, Celtic and Greek point to PIE *eḱs. Balto-Slavic *iś and *iź (either could be word-sandhi variant of the other) forms have unexplained initial *i-. However, due to aspirates in the old compounds such as ἔσχατος (éskhatos) and ἐχθός (ekhthós), a pre-form *h₁eǵʰs is needed.
- Albanian: ith (“back, backward”) (obsolete, present as part of bythë), ish (“former”) (< ek's)
- Celtic: *exs-
- Slavic: *jьz
- Beekes, Robert S. P. (2010) Etymological Dictionary of Greek (Leiden Indo-European Etymological Dictionary Series; 10), volume I, with the assistance of Lucien van Beek, Leiden, Boston: Brill, page 433