I’ve discussed this phenomenon in parts in places, but I have never full described the phenomenon of plūrālia tantum—Latin words that appear categorically as plural nouns.
The plūrālia include—
- names of cities: Athēnae, Thūriī, Philippī, Veiī
- names of festivals: Olympia, Bacchānālia, Quīnquārtrūs, lūdī Rōmānī
- names of social classes: optimātēs, maiōres (ancestors), liberī, penātēs, Quirītēs (citizens)
- words that are plural in nature, like the English ‘jeans, scissor, contents, etc.’: arma, artūs (joints), dīvitiae, scālae (stairs), valvae (folding doors), forēs, angustiae, moenia, dēliciae (beloved), faucēs (throat), īnsidiae (ambush), cervīcēs (neck), viscera (flesh).
- words that are popular plural poetical tropes: sceptra (for sceptrum), ora (for ōs), silentia (for silentium).
Where these appear in the singular, they often have meanings slightly distinct from their plural forms:
- Optimās, optimātis: aristocrat
- Foris, foris: gate
The Essential A & G: 101-2
Latin for Addicts 