Editing Latin declension (section)

==== {{anchor|d3i}} Third declension ''i''-stem and mixed nouns ====
The third declension also has a set of nouns that are declined differently. They are called '''''i''-stems'''. ''i''-stems are broken into two subcategories: pure and mixed. Pure ''i''-stems are indicated by special neuter endings. Mixed ''i''-stems are indicated by the double consonant rule. Stems indicated by the parisyllabic rule are usually mixed, occasionally pure.
;Masculine and feminine
:'''Parisyllabic rule:''' Some masculine and feminine third-declension ''i''-stem nouns have the same number of syllables in the genitive and the nominative. For example: {{wikt-lang|la|navis|nāvis, nāvis}} ('ship'); {{wikt-lang|la|nubes|nūbēs, nūbis}} ('cloud'). The nominative ends in ''-is'' or ''-ēs''.
:'''Double consonant rule:''' The rest of the masculine and feminine third-declension ''i''-stem nouns have two consonants before the ''-is'' in the genitive singular. For example: {{wikt-lang|la|pars|pars, partis}} ('part').
;Neuter
:'''Special neuter ending:''' Neuter third-declension ''i''-stems have no rule. However, all of them end in ''-al'', ''-ar'' or ''-e''. For example: {{wikt-lang|la|animal|animal, animālis}} ('animal'); {{wikt-lang|la|cochlear|cochlear, cochleāris}} ('spoon'); {{wikt-lang|la|mare|mare, maris}} ('sea').

The mixed declension is distinguished from the consonant type only by having ''-ium'' in the genitive plural (and occasionally ''-īs'' in the accusative plural). The pure declension is characterized by having ''-ī'' in the ablative singular, ''-ium'' in the genitive plural, ''-ia'' in the nominative and accusative plural neuter, and ''-im'' in the accusative singular masculine and feminine (however, adjectives have ''-em'').

The accusative plural ending ''-īs'' is found in early Latin up to [[Virgil]], but from the early empire onwards it was replaced by ''-ēs''.<ref name=":0">Gildersleeve & Lodge (1895), p. 18.</ref>

The accusative singular ending ''-im'' is found only in a few words: always in {{lang|la|tussis}} 'cough', {{lang|la|sitis}} 'thirst', {{lang|la|Tiberis}} 'River Tiber'; usually in {{lang|la|secūris}} 'axe', {{lang|la|turris}} 'tower', {{lang|la|puppis}} 'poop', {{lang|la|febris}} 'fever'; occasionally in {{lang|la|nāvis}} 'ship'. Most nouns, however, have accusative singular ''-em''.<ref>Gildersleeve & Lodge (1895), p. 27.</ref> The ending ''-im'' is not found in any adjectives, even those that have a separate feminine such as {{lang|la|celeris}} 'swift' or {{Lang|la|ācris}} 'keen',<ref>Ernout, A. (1953), {{lang|fr|Morphologie historique du latin}}, p. 50.</ref> or in any masculine common nouns.

The ablative singular ''-ī'' is found in nouns which have ''-im'', and also, optionally, in some other nouns, e.g. {{lang|la|in ignī}} or {{lang|la|in igne}} 'in the fire'.

There are two mixed-declension neuter nouns: {{wikt-lang|la|cor|cor, cordis}} ('heart') and {{wikt-lang|la|os|os, ossis}} ('bone'). The mixed declension is also used in the plural-only adjective  {{wikt-lang|la|plures|plūrēs, plūra}} ('most').

{| class="wikitable"
|+ Third declension paradigm<br />(''i''-stem nouns)
|-
! rowspan="2" |
! colspan="2" | Masculine &<br />Feminine || colspan="2" | Neuter
|-
! Singular || Plural
! Singular || Plural
|-
! Nominative, Vocative
|— ||-ēs || rowspan="2" | — || rowspan="2" | -ia
|-
! Accusative
| -em<br />-im || -ēs<br />-īs
|-
! Genitive
| -is || -ium || -is || -ium
|-
! Dative
| -ī || rowspan="2" | -ibus || rowspan="2" | -ī || rowspan="2" | -ibus
|-
! Ablative
| -e<br />-ī
|-
! Locative
| ''Dat./Abl.'' || ''Dat.'' || ''Dat./Abl.'' || ''Dat.''
|}

{| class="wikitable"
|+ Third declension paradigm<br />(mixed nouns)
|-
! rowspan="2" |
! colspan="2" | Masculine &<br />Feminine || colspan="2" | Neuter
|-
! Singular || Plural
! Singular || Plural
|-
! Nominative, Vocative
|— ||-ēs || rowspan="2" | — || rowspan="2" | -a
|-
! Accusative
| -em || -ēs<br />-īs
|-
! Genitive
| -is || -ium || -is || -ium
|-
! Dative
| -ī || rowspan="2" | -ibus || -ī || rowspan="2" | -ibus
|-
! Ablative
| -e || -e
|-
! Locative
| ''Dat./Abl.'' || ''Dat.'' || ''Dat./Abl.'' || ''Dat.''
|}

{| class="wikitable"
! rowspan="3" |
! colspan="2" | {{wikt-lang|la|turris|turris, turris}}<br />tower {{abbr|f.|feminine}} (pure)
! colspan="2" | {{wikt-lang|la|pars|pars, partis}}<br />part, piece {{abbr|f.|feminine}} (mixed)
! colspan="2" | {{wikt-lang|la|animal|animal, animālis}}<br /> animal, living being {{abbr|n.|neuter}} (pure)
|-
! colspan="2" | Parisyllabic rule
! colspan="2" | Double consonant rule
! colspan="2" | Special neuter ending
|-
! Singular
! Plural
! Singular
! Plural
! Singular
! Plural
|-
! Nominative,  Vocative
|turris||turrēs ||pars ||partēs || rowspan="2" | animal || rowspan="2" | animālia
|-
! Accusative
| turrem<br />turrim || turrēs<br />turrīs || partem || partēs<br />partīs
|-
! Genitive
| turris || turrium || partis || partium || animālis || animālium
|-
! Dative
| turrī || rowspan="2" | turribus || partī || rowspan="2" | partibus || rowspan="2" | animālī || rowspan="2" | animālibus
|-
! Ablative
| turre<br />turrī || parte<br />(partī)
|}

The rules for determining ''i''-stems from non-''i''-stems and mixed ''i''-stems are guidelines rather than rules: many words that might be expected to be ''i''-stems according to the parisyllabic rule actually are not, such as {{wikt-lang|la|canis}} ('dog') or {{wikt-lang|la|iuvenis}} ('youth'), which have genitive plural {{lang|la|canum}} 'of dogs' and {{lang|la|iuvenum}} 'of young men'. Likewise, {{wikt-lang|la|pater}} ('father'), {{wikt-lang|la|māter}} ('mother'), {{wikt-lang|la|frāter}} ('brother'), and {{wikt-lang|la|parēns}} ('parent') violate the double-consonant rule. This fluidity even in Roman times resulted in much more uncertainty in Medieval Latin.

Some nouns in ''-tāt-'', such as {{wikt-lang|la|civitas|cīvitās, cīvitātis}} 'city, community' can have either consonant-stem or ''i''-stem genitive plural: {{lang|la|cīvitātum}} or {{lang|la|cīvitātium}} 'of the cities'.<ref name=":0"/>