In Ancient Greek, all nouns are classified according to grammatical gender (masculine, feminine, neuter) and are used in a number (singular, dual, or plural).
According to their function in a sentence, their form changes to one of the five cases (nominative, vocative, accusative, genitive, or dative).
The thematic vowel (ο or ᾱ) counts as neither stem nor ending, but alternates between the two depending on which accent is considered.
Homer retains the older masculine ending -ᾱ and uses ναύτᾱ "sailor" instead of ναύτης: compare Latin nauta.
In the Attic dialect, some masculine second-declension nouns and some adjectives have endings with lengthened vowels.
Some nouns in this category end in -εως, which developed from an original *-ηος by the process of quantitative metathesis (switching of vowel lengths).
All second-declension endings containing ο were transformed: The placement of the accent does not change, even when the ultima is long, and all forms take an acute instead of a circumflex.
When the ultima is accented, it takes a circumflex in all forms, including the nominative, accusative, and vocative.
This results in varied and often complex phonemic interactions between stem and ending, especially so between adjacent consonants, that often make these nouns appear to be highly irregular compared to their straightforward thematic counterparts.
These nouns in the nominative singular end with the vowels α, ι, υ, ω or with the consonants ν, ρ, ς (ξ, ψ).
When a Proto-Greek consonant was lost (ϝ, ι̯, σ), -α appears after a vowel, and may be lengthened to ᾱ: βασιλέᾱ.
A peculiar subset of this declension is used when declining foreign masculine names such as Ἰησοῦς, Ἰούδᾱς, Λευῑ́(ς): the nominative takes a sigma, the genitive and vocative are the bare stem, the dative may receive an iota subscript if possible, the accusative receives a nu.
Nominative singular -ς and dative plural -σι cause pronunciation or spelling changes, depending on the consonant at the end of the stem.
In the nominative singular and dative plural, a dental τ, δ, θ before σ is lost: τάπης, not τάπητς.
In the vocative singular, final -τ is lost, as Ancient Greek words cannot end in stops.
Nouns of this class that are not accented on the last syllable use the weak stem without an ending for the vocative singular.
Unlike mute-stems, these nouns do not change in spelling or pronunciation when the dative plural ending -σι is added.
Nouns in this class that are not accented on the last syllable use the weak stem without an ending for the vocative singular.
The α in the dative plural was added for ease of pronunciation; the original form ended in -ρσι.
The long-vowel stem in the genitive singular was shortened, and the vowel in the ending lengthened (quantitative metathesis).
Both originally ended with digamma, which by the time of Classical Greek had either vanished or changed to υ.
In Attic Greek the η of the stem underwent quantitative metathesis with the vowel of the ending—the switching of their lengths.
The nouns with a vowel before the -εύς often contract the final ε of the stem (either original or from quantitative metathesis of η), which disappears into the following ω and ᾱ of the genitive and accusative singular and plural.
As is the rule, the vowel resulting from contraction takes a circumflex: Stems in -οι- end in -ω in the nominative singular.