Appendix:Ancient Greek third declension

The Ancient Greek third declension (also known as the consonant declension) comprises the most diverse and potentially confusing forms of nominal inflection. The third declension does not have a stem vowel, as the first (α/η) and second (ο) declensions do. Since the stem vowels provide a sort of buffer between the stems and inflectional endings, the third declension is more prone to contractions and other irregularities than the other two.

Etymology
The third declension is the Ancient Greek reflex of the Proto-Indo-European (PIE) consonant declension. A dash with nothing following indicates an unmarked inflection, that is, a bare stem.

† The neuter nominative and accusative in the singular and plural have a different inflection from the masculine and feminine.

Basic pattern
In Ancient Greek the ablative and genitive have merged into a single case, a process already well underway in Proto-Indo-European. The dative, locative, and instrumental cases were all merged into the dative, which generally takes on the inflection of the locative. Consult the Wikipedia article on Indo-European sound laws to aid in understanding some of the Ancient Greek reflexes of PIE sounds, such as the perhaps unintuitive evolution of PIE *m̥ &rarr; Ancient Greek α.

Stop consonants
In nouns which end with a labial (π, β, φ, ψ) or velar (κ, γ, χ, ξ) consonant, the pattern is fairly recognizable, as seen in. Note how the γ and σ merge into a ξ in the nominative singular and dative plural.

Nouns which end with a dental (τ, δ, θ) show some contractions. Ancient Greek does not allow a δσ, θσ, or τσ combination, and so the dental is generally dropped, often with a lengthening of the stem vowel, which sometimes goes back to PIE itself. Compare the inflection of, stem , with that given for.

Neuters
Neuter nouns of the third declension are nearly identical to their masculine and feminine counterparts except for the nominative, accusative, and vocative cases in the singular and plural. Since the stem often ends with a sound which an Ancient Greek word cannot end on, the final sound is often dropped or changed in unmarked forms. The simplest and most common third declension neuters are the dental stems, such as, stem. Interestingly, the τ in the stem is a common feature of Ancient Greek words derived from PIE neuter n stems, which is not well explained.

Contractions
Nouns with stems ending in ι show an odd admixture of ι and ει stem endings. Homeric Greek shows a more consistent ι ending resulting in εις in the plural.   Take the example of :

Neuter stems ending with a Vσ (where V is a vowel) drop the sigma, and subsequently contract, especially in later Greek. The inflection of, stem is as follows: