“So which is correct?” asked a student. “θάλαττα or θάλασσα? Is it to do with dialects?”
Both are correct, and both belong to the Attic dialect. θάλαττα, and other words with the double τ, represent a colloquial form, which found its way into Aristophanes, Plato, and the orators; but the double σ, in this and many other words, was kept by the tragedians and by Thucydides and Xenophon, and was permanently retained.
The formal Attic dialect later became the usage of Greek adopted by Alexander the Great, and was the official language of all the cities he founded. And so Attic Greek became the common tongue for the whole of the Eastern Mediterranean: ἡ κοινὴ διάλεκτος. The New Testament is written in a simple form of it, but with about 10% of words not found in the classical era.
The OED definition of a dialect:
‘A form or variety of a language which is peculiar to a specific region, esp. one which differs from the standard or literary form of the language in respect of vocabulary, pronunciation, idiom, etc.’
We know too little about the other dialects of Greek, because so few works in them survive. The Aeolic dialect is to be found in the works of Alcaeus and Sappho. As far as Doric is concerned, we have Theocritus, and fragments of other writers. (The choruses of Attic tragedies, which are supposed to be in the Doric dialect, represent only a conventional literary form of Doric, which is mainly to do with pronunciation.)
For those students who are interested, there are three chief dialects of classical Greek:
Ionic: spoken by Ionians in Asia Minor, Attica, and numerous islands and colonies. The Ionic dialect was the first to become a literary one. Its subdivisions are:
- Old Ionic, or Epic, found in Homer and Hesiod;
- New Ionic, found in Herodotus;
- Attic, found in all the Athenian writers.
Aeolic, spoken by the Aeolians in Asia Minor, Boeotia and Thessaly, and used by Alcaeus and Sappho.
Doric, spoken by the Dorians in the Peloponnese, Northern Greece, Crete, and many colonies in Sicily and southern Italy. Theocritus—born in Sicily, perhaps Syracuse—wrote mainly in Doric.