The Vocative Singular of βασιλεύς...

[Lex]

...is βασιλεῦ. Why the circumflex? I don't find a rule in Pharr that covers this. Is there such a rule, or is this one of those things you just have to memorize?

[annis]

For this, memorize. Even Chandler throws up his arms for third declension nouns —

580. The irregularities of these substantives are so great that it is impossible to give any general rule which can be depended on, but it may perhaps assist the memory to mention the main result of the special rules subjoined. Speaking then in the widest terms, and neglecting the thousand and one exceptions which occur, ...

[Lex]

Thanks, William.