Reconstruction:Proto-Slavic/sǫ-

From Proto-Balto-Slavic *sam-, from Proto-Indo-European *som (Derksen) or *sem- (“together, one”).

Baltic cognates include Lithuanian sam-, Latvian suo-, Old Prussian sen-.

Indo-European cognates include Sanskrit सम् (sam, “together, at the same time”), Avestan 𐬵𐬀𐬨‎ (ham-), 𐬵𐬀‎ (ha-), Old Persian [script needed] (ham-), Ancient Greek ὁμοῦ (homoû, “together”), ἅμα (háma, “together with”), ἁ- (ha-), Latin semel (“once, a single time”).

*sǫ-

1. (Nominal prefix) together

• *sъ(n), *sъ(n)- (“from, with”)

• East Slavic:
  • Belarusian: су- (su-)
  • Russian: су- (su-), со- (so-)
  • Ukrainian: су- (su-)
• South Slavic:
  • Old Church Slavonic:

    Old Cyrillic: сѫ- (sǫ-)

    Glagolitic: ⱄⱘ- (sǫ-)

  • Bulgarian: съ- (sǎ-)
  • Serbo-Croatian:

    Cyrillic: су-

    Latin: su-

  • Slovene: so-
• West Slavic:
  • Czech: sou-
  • Polish: są-
  • Slovak: sú-
  • Sorbian:
    • Upper Sorbian: su-
    • Lower Sorbian: su-

• Derksen, Rick (2008) Etymological Dictionary of the Slavic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 4), Leiden, Boston: Brill, →ISBN, page 462
• Vasmer (Fasmer), Max (Maks) (1964–1973), “су-”, in Etimologičeskij slovarʹ russkovo jazyka [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Trubačóv Oleg, Moscow: Progress
• Černyx, P. Ja. (1999), “су-”, in Istoriko-etimologičeskij slovarʹ russkovo jazyka [Historical-Etymological Dictionary of the Russian Language] (in Russian), volume 2, 3rd reprint edition, Moscow: Russkij jazyk, page 215