Lexical usage

Specific place names

Ἀθήναι or Ἀθήνησι(ν) ‘in Athens’ (on inscriptions), Δελφοῖς ‘in Delphi’, Ἰσθμοῖ ‘on the Isthmus’, Μεγαροῖ ‘in Megara’, Ὀλυμπίᾱσι(ν) ‘in Olympia’, Πλαταιαῖς ‘in Plataiai’, Πῡθοῖ ‘in Pytho’, Σαλαμῖνι ‘in Salamis’ etc. The same place names also occur with ἐν, for instance ἐν Ὀλυμπίᾳ and ἐν Πλαταιαῖς. With names of countries and places ἐν is more common than the simple dative in Attic prose.
As a rule, the names of Attic demes are in the locative, for example Φαληροῖ, Θορικοῖ or Μαραθῶνι. Some names of demes use ἐν, for example ἐν Κοίλῃ or ἐν Μαραθῶνι.

Common nouns (only in poetry)

This often concerns parts of the body (e.g. θῡμῷ or καρδίῃ in Homer). If the word in the dative is a person, it is usually in the plural. Besides parts of the body, common names denoting a physical location (e.g. αἰθέρι ‘in the air’, οὔρεσι(ν) ‘in the mountains’, πόντῳ ‘on sea’) or social gatherings (e.g. ἀγορῇ ‘in the council’, μάχῃ ‘in battle’) are also put in the dative.

Historical background

Even in Homer the use of the dative without a preposition to signal a place is not very common. In classical Greek the preposition ἐν + dative definitely becomes the norm.