The dual number refers to exactly two people or things. It's relatively rare, surviving mainly in Classical Attic and earlier Greek.
Common dual uses: • Pairs of body parts: τὼ χεῖρε (the two hands), τὼ ὀφθαλμώ (the two eyes) • Two people acting together: τὼ ἄνδρε (the two men) • Natural pairs: the Dioscuri (Castor and Pollux)
Dual forms: • Nominative/Accusative: -α (1st decl), -ω (2nd decl), -ε (3rd decl) • Genitive/Dative: -αιν (1st decl), -οιν (2nd and 3rd decl)
Dual verb endings: -τον (2nd person; 3rd person in primary tenses), -την (3rd person in secondary tenses)
The dual became obsolete in Koine Greek.