Zethus was the twin brother of Amphion, both being the sons of Zeus and Antiope and joint rulers of Thebes. As for Zetus, Euripides connected the name with zeteo, 'to seek', 'search out', although if we interpret Amphion's name as 'he who goes round', from amphi, 'round' (see names starting with this), and regard him as representing the sun, then Zethus could have a name originating in zeo, 'to boil', 'heat', and share his twin's representation. The idea is fanciful, of course, yet the brothers' mother was Antiope, whose 'face opposite' name could allude to the moon - and the twins were the sons of the great sky-god Zeus!