Herbert Weir Smyth, A Greek Grammar for Colleges

The instrument or means of an action is expressed by the primary suffixes

1. τρο (nom. -τρο-ν, neut.): ἄρο-τρο-ν plough (ἀρό-ω plough), λύ-τρο-ν ransom (λύ_-ω release, λυ_^-), σεῖ-σ-τρο-ν rattle (σεί-ω shake, 624 a), δί-δακ-τρο-ν teacher's pay (διδάσκω teach, διδαχ-), λου-τρό-ν bath (bathing-water; λού-ω wash).

2. θ-ρο (nom. -θρο-ν, neut.): κλεῖ-θρο-ν bar for closing a door (κλεί-ω shut, 832).

3. τρα_ (nom. -τρα_, fem.): μάκ-τρα_ kneading-trough (μάττω knead, μαγ-), ῥή-τρα_ compact (ἐρέω ἐρῶ shall say, ἐρ-, ῥε-), χύ-τρα_ pot (χέω pour, χυ-).

4. τηρ-ιο- (nom. -τηρ-ιο-ν, neut.): in a few words, as πο-τήρ-ιο-ν cup (πί_νω drink, πο- 529); θελκ-τήρ-ιο-ν spell, charm (θέλγ-ω charm). See 858. 14.

5. ειο (rare; nom. -εῖον, neut.): τροφεῖα pay for rearing. See 863 a. 8.

6. ρο (nom. -ρό-ν, neut.): πτ-ε-ρό-ν wing (πέτ-ομαι fly).