Proto-Slavic *sǫ-

→ Belarusian сусвет (affix)

→ Belarusian супраць (affix)

→ Czech sou- (inherited)

→ Old Church Slavonic сѫдии (mention)

→ Old Church Slavonic сѫ- (inherited)

→ Polish sąsiad (mention)

→ Polish są- (inherited)

→ Russian соперник (derived)

→ Russian су- (inherited)

→ Russian су- (mention)

→ Slovak sú- (inherited)

→ Ukrainian суперник (derived)