Appendix:Latin third declension

Nouns

Description

Latin nouns of the third declension have genitive singular forms in -is. The nominative and vocative singular forms are identical to each other, and are often quite different from other cases: thus it is common to refer to third-declension nouns using two citation forms, the nominative singular and genitive singular, separated by a comma, e.g. rādīx, rādīcis f (“root”). This declension includes nouns of all three genders.

The third declension includes both nouns that originally had a stem ending in a consonant, and nouns that originally had a stem ending in the vowel i (as seen for example in the neuter genitive plural animalium). By Classical Latin, these two etymologically distinct categories had become confused, and in a number of cases, conflated: as a result, in addition to 'pure' consonant-stem and i-stem inflection, many third-declension nouns display 'mixed' patterns of declension.

Words borrowed from Greek's third declension are inflected with a varying mixture of Greek and Latin case endings.

General endings

**Third declension endings**
Case	Singular		Plural
Case	m./f.	n.	m./f.	n.
nominative	-s / -ēs / -is	-∅ -e	-ēs	-a -ia
genitive	-is		-um -ium
dative	-ī		-ibus
accusative	-em (-im)	-∅ -e	-ēs (-īs)	-a -ia
ablative	-e (-ī)		-ibus
vocative	-s / -ēs / -is	-∅ -e	-ēs	-a -ia
locative	-ī (-e)		-ibus

The nominative singular often exhibits complex sound changes between the stem and the suffix, and so students of Latin are typically taught to memorize this entire case-form as a stem of its own. Non-neuter nouns normally end in -s in the nominative singular, which combines with a preceding -c- /k/ or -g- /ɡ/ to form -x /ks/, as in vōx, vōcis or rēx, rēgis. Other endings can also be seen, such as -ō in non-neuter n-stem nouns (e.g. homō, hominis).
Certain endings have consonant-stem and i-stem variants. I-stem nouns regularly have a nominative/accusative neuter plural in -ia and a genitive plural in -ium. Masculine and feminine i-stem nouns commonly formed their accusative plural in -īs up through Classical Latin, but it eventually became common in later Latin to replace this with the variant form -ēs. The ablative singular in -ī is found in i-stem neuter nouns and a relatively small number of "pure" i-stem masculine or feminine nouns. The accusative singular in -im is also a feature of "pure" non-neuter i-stems. Most non-neuter i-stem nouns instead display a 'mixed' declension pattern with singular forms like those of a consonant stem (an ablative singular in -e and an accusative singular in -em) alongside i-stem plural forms.
Locative forms exist for city names and some island names, but not for the vast majority of common nouns (which are used with a preposition instead to convey the same sense).
- The locative singular may end either in -ī, as in Carthāginī, or in -e, as in Carthāgine.
- Among the few locative forms derived from common nouns of the third declension are rūrī (“in the countryside”) from rūs (“the countryside”), temporī (“at the right time, seasonably”) from tempus (“time”), and lūcī (“by daylight, in daylight”) from lūx (“light”); in the latter two the locative has a temporal rather than a spatial sense.
- As in all declensions, locative plural forms (used for plurale tantum proper nouns) use the same endings as the plural dative/ablative: thus, the city-name Trallēs uses the form Trallibus for its locative,^[1] and Gādēs uses Gādibus.

Non-neuter nouns

Nouns are called "parisyllabic" if they have the same number of syllables in the nominative and genitive singular (e.g. the nom. civis and gen. civis both have two syllables). Nouns are called "imparisyllabic" if they have different numbers of syllables in the nominative and genitive singular (e.g. the nom. consul and gen. consulis have two and three syllables, respectively).

Parisyllabic masculine or feminine third-declension nouns tend to show i-stem endings in the plural: genitive plural -ium and optionally accusative plural -īs. I-stem endings are less common in the singular.

Parisyllabic nouns with a nominative singular in -is can show several different patterns of inflection in the singular. A few, said to have 'pure' i-stem inflection, form their accusative singular in -im and their ablative singular in -ī. This includes:

some feminine common nouns that are parisyllabic and end in -is (always būris f, cucumis f, rāvis f, sitis f, tussis f; sometimes febris f, puppis f, restis f, turris f, secūris f, sēmentis f; rarely others); also the irregularly declined noun vīs f.^[2]
names of rivers (which are typically masculine) that are parisyllabic and end in -is, such as Tiberis m, Fabaris m.
some nouns borrowed from Greek (see sections below).

Others generally form their accusative singular in -em, such as cīvis m or f (accusative cīvem). Nouns with an accusative singular in -em usually form their ablative singular in -e, but for certain nouns this form usually or occasionally ends in -ī instead.^[2]

Imparisyllabic masculine or feminine third-declension nouns form their accusative singular in -em and their ablative singular in -e. In the plural, many take the consonant-stem endings -um (genitive plural) and -ēs (accusative plural). Others show what is called "mixed i-stem" declension, taking the endings -em and -e in the singular but -ium (and optionally, accusative -īs) in the plural.

Imparisyllabic nouns with an oblique stem that ends in two or more consonants (or alternatively, that have a nominative singular that ends in a consonant followed by -s or -x) usually show "mixed" declension: e.g. dēns (ablative singular dente, genitive plural usually dentium), falx (ablative singular falce, genitive plural falcium).
Some imparisyllabic nouns with an oblique stem that ends in only one consonant nevertheless have a genitive plural in -ium: e.g. līs f (genitive plural lītium), mūs m or f (genitive plural usually mūrium), dōs f (genitive plural dōtium or dōtum), etc.

Examples

Imparisyllabic consonant stem

Citation form: homō, hominis m

Case	Singular	Plural
nominative	homō	homin·ēs
genitive	homin·is	homin·um
dative	homin·ī	homin·ibus
accusative	homin·em	homin·ēs
ablative	homin·e	homin·ibus
vocative	homō	homin·ēs

Parisyllabic i-stem

Citation form: turris, turris f

Case	Singular	Plural
nominative	turris	turr·ēs
genitive	turr·is	turr·ium
dative	turr·ī	turr·ibus
accusative	turr·em (-im)	turr·ēs (-īs)
ablative	turr·e (-ī)	turr·ibus
vocative	turris	turr·ēs

Mixed declension (mixed i-stem)

Citation form: nox, noctis f

Case	Singular	Plural
nominative	nox	noct·ēs
genitive	noct·is	noct·ium
dative	noct·ī	noct·ibus
accusative	noct·em	noct·ēs
ablative	noct·e	noct·ibus
vocative	nox	noct·ēs

Greek-type variants

Some third-declension nouns taken from Greek retained forms based on Greek declension in at least some cases (sometimes as an alternative to more Latinized forms). Greek endings can be seen for a number of nouns in the singular and plural forms of the nominative, accusative, and vocative, as well as the genitive singular, marked in Greek by -ος (-os); they are seen less frequently for the genitive plural, marked in Greek by -ων (-ōn), or the dative plural, marked in Greek by -σῐ (-sĭ) or -σῐν (-sĭn).

Greek consonant-stem endings

Consonant-stem nouns taken from Greek's third declension sometimes retain the following Greek endings in Latin texts:

Cases	Singular		Plural
Nom.	~(s)/~(ēr)/~(n)	~(ys)	-es
Gen.	-os		(-ōn)
Dat.	(-i)		(-si, -sin)
Acc.	-a		-as
Abl.	—		—
Voc.	~(s)/~(ēr)/~(n)	(~ys), -y	-es

Examples:

Greek-type nominative plural in -ĕs: lampades, Telchīnes
Greek-type accusative plural in -ăs: lampadas, lebētas

Latin endings often can be used instead.

The genitive singular can be -is, as in Latin, or -os, as in Greek; the latter is found especially in words with a genitive singular in -dis/-dos.
The genitive plural can be -um, as in Latin: e.g. delphīnum (used by Virgil in the Aeneid alongside the Greek-type nominative plural delphīnes).
In the dative/ablative plural, the Latin ending -ibus is usually used (e.g. hērōibus, Cicero). Examples of words where a Greek-type dative plural is attested include hērōsin (Ovid) from ἥρωσῐν (hḗrōsĭn) and ēthesin (Varro) from ἤθεσῐν (ḗthesĭn). "herosi" can be found in Prisciani Caesariensis Grammatici Opera ("non herosi [...] herosi").

Examples:

hērōs, adamās (also adamāns)
delphīn (also delphīs)
pēlamys, chlamys from πηλᾰμῠ́ς (pēlămŭ́s), χλᾰμῠ́ς (khlămŭ́s)
crātēr, āēr, lampas from κρᾱτήρ (krātḗr), ᾱ̓ήρ (āḗr), λαμπάς (lampás)

lampas, lampadis (“lamp”) f
Case	Singular	Plural
nominative	lampas	lampad·es
genitive	lampad·is / lampad·os	lampad·um
dative	lampad·ī	lampad·ibus
accusative	lampad·a / lampad·em	lampad·as
ablative	lampad·e	lampad·ibus
vocative	lampas	lampad·es

crātēr, crātēris (“basin, bowl”) m
Case	Singular	Plural
nominative	crātēr	crātēr·es
genitive	crātēr·is	crātēr·um
dative	crātēr·ī	crātēr·ibus
accusative	crātēr·a / crātēr·em	crātēr·as
ablative	crātēr·e	crātēr·ibus
vocative	crātēr	crātēr·es

Greek i-stems

Words like basis tend to take the same or similar endings as Latin i-stem nouns. Some Greek endings may be used, such as -in in the accusative singular or -eōs in the genitive singular.

Case	Singular	Plural
Nom.	-is	-ēs
Gen.	-is; -eos, -ios	-ium; -eōn
Dat.	-ī	-ibus
Acc.	-im; -in	-īs, -ēs
Abl.	-ī	-ibus
Voc.	-is (-i)	-ēs

Vowel lengths: Gen. sg.: In older English works (A&G, L&S) it is -eōs instead of -eos. Allen & Greenough and Lewis & Short have -eōs; Lewis Marcus, Georges, Pons have -eos. As Georges and Pons are newer than A&G and L&S, it seems more reliable.
Examples: haeresis, -eos f from αἵρεσῐς, -ρέσεως/-ρέσῐος f (haíresĭs, -réseōs/-résĭos); basis, basis/baseos f (acc. pl. also -e͡is and according to Lewis Marcus with other irregular forms) from βᾰ́σῐς, βᾰ́σεως (bắsĭs, bắseōs), poēsis from ποίησις (poíēsis)

Some words with a nominative in -is, such as tigris, tigridis/tigris m or f, can have an alternative stem in -id-.

Citation form: tigris, tigridis m and f

Case	Singular	Plural
nominative	tigris	tigr·ēs / tigrid·es
genitive	tigrid·is / tigr·is / tigrid·os	tigr·ium
dative	tigrid·ī / tigr·ī	tigr·ibus
accusative	tigr·im / tigr·in	tigr·is / tigrid·as
ablative	tigrid·e / tigr·ī	tigr·ibus
vocative	tigris	tigr·ēs / tigrid·ēs

Greek y-stems

Like chelys f, from χέλῡς/χέλῠς (khélūs/khélŭs):

Accusative singular -yn, as in chelyn. Partially Latinized accusative forms in -ym are sometimes found, such as cac(h)rym from κᾰ́χρῠν (kắkhrŭn).
Vocative singular -y, as in chely.
Genitive singular -yis, as in Cotyis, or -yos, as in cachryos.
Other forms of chelys f are unattested. Instead of e.g. -ibus in dat.&abl. pl. it might also be chelybus (Lewis Marcus).

Greek -οι-stems

Nominative singular in -ō, like ēchō f from ἠχώ (ēkhṓ). This -ō was always long, unlike the nominative/vocative singular of Latin third-declension nouns such as virgō̆, which eventually came to have short -ŏ.
Genitive singular in -ūs. Since this genitive singular ending is characteristic of the fourth declension in Latin, such nouns are sometimes categorized as belonging to that declension (see Appendix:Latin fourth declension), even though they are third-declension in Greek.
Other singular cases -ō. But Georges mentions acc. sg as -ōn for ēchō. It might also be -ōnis, -ōnī, -ōnem, -ōne (gen., dat., acc., abl. sg.), like the proper noun Dīdō, which was sometimes adapted and inflected like a native Latin third-declension word with a genitive singular in -ōnis.
Not used in the plural.

Neuter nouns

Neuter nouns follow a different rule. Rather than counting the syllables, neuter third-declension nouns tend to be pure i-stem if the nominative singular ends either in -e, or in -al or -ar with a genitive singular in -ālis/-āris. Otherwise, neuter third-declension nouns tend to be consonant-stems.

Neuter consonant stem

Neuter consonant-stem nouns end in -um in the genitive plural, -a in the nominative/accusative/vocative plural, and -e in the ablative singular. Most are either n-stem nouns or historical s-stem nouns. N-stem neuter nouns inflect like nōmen, nōminis. Historical s-stem neuter nouns generally inflect like either genus, generis or corpus, corporis, with -r- in the oblique cases due to rhotacism of intervocalic -s-.

nōmen, nōminis (“name”) n
Case	Singular	Plural
nominative	nōmen	nōmin·a
genitive	nōmin·is	nōmin·um
dative	nōmin·ī	nōmin·ibus
accusative	nōmen	nōmin·a
ablative	nōmin·e	nōmin·ibus
vocative	nōmen	nōmin·a

genus, generis (“type, kind”) n
Case	Singular	Plural
nominative	genus	gener·a
genitive	gener·is	gener·um
dative	gener·ī	gener·ibus
accusative	genus	gener·a
ablative	gener·e	gener·ibus
vocative	genus	gener·a

corpus, corporis (“body”) n
Case	Singular	Plural
nominative	corpus	corpor·a
genitive	corpor·is	corpor·um
dative	corpor·ī	corpor·ibus
accusative	corpus	corpor·a
ablative	corpor·e	corpor·ibus
vocative	corpus	corpor·a

Less frequently, neuter third-declension nouns can have a stem ending in l, as in mel, mellis n (“honey”), or r (not from underlying s), as in far, farris n (“emmer”).

Only a handful of native Latin neuter nouns have a stem that ends in a consonant other than n, s, l or r:

caput, capitis n (“head”) is inflected throughout with consonant-stem endings, such as capite, capita, capitum. (The ablative singular has a rare alternative form capitī.)
lac, lactis n (“milk”) has the consonant-stem ablative singular lacte. It is not used in the plural.
cor, cordis n (“heart”) has the consonant-stem ablative singular corde and nominative/accusative/vocative plural corda. Its genitive plural is unattested in Classical Latin, but appears as i-stem cordium in the Vulgate.
os, ossis n (“bone”) has the consonant-stem ablative singular osse and nominative/accusative/vocative plural ossa. Its genitive plural can be i-stem ossium or alternatively u-stem ossuum (the nominative/accusative/vocative also has an alternative u-stem form ossua).

Neuter i-stem

Neuter i-stem nouns end in -ium in the genitive plural, -ia in the nominative/accusative/vocative plural, and generally -ī in the ablative singular. (Some have alternative ablative singular forms ending in -e.) The nominative singular ends either in -e, in -al with a genitive singular in -ālis, or in -ar with a genitive singular in -āris. In this context, the endings -al and -ar developed by syncope from -āle and -āre, the neuter versions of the adjective-forming suffix -ālis and its variant form -āris.

missile, missilis (“projectile”) n
Case	Singular	Plural
nominative	missile	missil·ia
genitive	missil·is	missil·ium
dative	missil·ī	missil·ibus
accusative	missile	missil·ia
ablative	missil·ī	missil·ibus
vocative	missile	missil·ia

animal, animālis (“animal”) n
Case	Singular	Plural
nominative	animal	animāl·ia
genitive	animāl·is	animāl·ium
dative	animāl·ī	animāl·ibus
accusative	animal	animāl·ia
ablative	animāl·ī	animāl·ibus
vocative	animal	animāl·ia

calcar, calcāris (“spur”) n
Case	Singular	Plural
nominative	calcar	calcār·ia
genitive	calcār·is	calcār·ium
dative	calcār·ī	calcār·ibus
accusative	calcar	calcār·ia
ablative	calcār·ī	calcār·ibus
vocative	calcar	calcār·ia

Other nouns declined like those above include equīle, equīlis; sollemne, sollemnis; vectīgal, vectīgālis; tribūnal, tribūnālis; exemplar, exemplāris; pulvīnar, pulvīnāris.

The noun mare (“sea”) ends in -ia in the nominative/accusative/vocative plural (maria) and usually ends in -ī in the ablative singular, but mare is also attested as an alternative ablative singular form. The i-stem form of its genitive plural is not attested in Classical Latin. The noun rēte (“net”) tends to end in -e rather than -ī in the ablative singular. The nouns iubar, iubaris and baccar, baccaris, which end in -ăris rather than -āris in the genitive singular, do not decline as i-stems but as consonant stems.

Neuter Greek nouns

The most numerous category of neuter third-declension nouns taken from Greek was nouns in -ma with a stem in -mat-, such as poēma from Greek ποίημᾰ n (poíēmă), genitive ποιήμᾰτος (poiḗmătos). These take Latin third-declension consonant-stem endings in the singular, but sometimes take second-declension endings in the plural (especially the dative/ablative plural):

poēma, poēmatis (“poem”) n
Case	Singular	Plural
nominative	poēma	poēmat·a
genitive	poēmat·is	poēmat·um poēmat·ōrum
dative	poēmat·ī	poēmat·īs poēmat·ibus
accusative	poēma	poēmat·a
ablative	poēmat·e	poēmat·īs poēmat·ibus
vocative	poēma	poēmat·a

A small number of Greek neuter third-declension nouns ending in -ος, such as ἔπος (épos), were taken into Latin. Often only some cases are attested in ancient times; e.g. epos used only in the nominative and accusative singular; chaos used in Classical Latin only in the nominative/accusative singular and ablative singular chaō. The endings of the Latin second declension were also used to adapt nouns like this, sometimes with a change in gender to the masculine (e.g. cētus m from κῆτος n (kêtos)). Some Greek contracted neuter plurals in -η, such as μέλη, from μέλος (mélos), were taken into Latin as rare neuter plural forms in -ē, e.g. melē, cētē = κήτη.

Adjectives

The third declension includes some positive adjectives and all comparative adjectives.

Positive

Third-declension positive adjectives can be divided into the following three categories:

two terminations: these adjectives have two distinct nominative singular forms: one ending in -is used for the masculine and feminine, and one ending in -e for the neuter.
three terminations: these adjectives have three distinct nominative singular forms: the masculine ends in -er, the feminine ends in -ris, and the neuter ends in -re.
one termination: these adjectives have only one nominative singular form shared between all genders. Present participles are included in this category.

Positive adjectives of two or three terminations regularly take i-stem endings in the ablative singular (-ī), genitive plural (-ium), and neuter nominative/accusative/vocative plural (-ia), and optionally in the masculine/feminine accusative plural (-īs, alongside consonant-stem (-ēs). Adjectives of one termination may show either i-stem or consonant-stem forms.

Two-termination (i-stem)

This category contains the majority of positive third-declension adjectives. A number of common suffixes form adjectives of two terminations, such as -īlis, -īle; -ālis, -āle; -āris, -āre; and -ēnsis, -ēnse.

Citation form: trīstis, trīste

Case	m f singular	n singular	m f plural	n plural
nominative	trīst·is	trīst·e	trīst·ēs	trīst·ia
genitive	trīst·is		trīst·ium
dative	trīst·ī		trīst·ibus
accusative	trīst·em	trīst·e	trīst·ēs, -īs	trīst·ia
ablative	trīst·ī		trīst·ibus
vocative	trīst·is	trīst·e	trīst·ēs	trīst·ia

Three-termination (i-stem)

This is a small category. The only difference from adjectives of two terminations is the masculine nominative/vocative singular form ending in -er. Often a masculine nominative/vocative singular form ending in -r·is is also attested: that is, many third-declension adjectives of three terminations can alternatively be declined as adjectives of two terminations. In addition, the nominative singular forms ending in -er are sometimes attested in the feminine.

The genitive plural typically uses the i-stem ending -ium, but a few words in this category have an attested genitive plural form with the consonant-stem ending -um. These consonant-stem forms are most often seen in contexts where the word is being used as a non-neuter noun, rather than as an adjective: for example, celerum occurs as the genitive of the masculine noun celerēs (the name of a certain Roman military unit), and volucrum occurs as the genitive of the feminine noun volucrēs (“birds, flying creatures”). However, some adjectival uses are also attested: volucrum can modify a masculine or feminine noun in poetry, functioning as a genitive plural of the adjective volucer, and celerum is used as an adjective by the Late Latin medical writer Caelius Aurelianus (modifying the feminine noun passiōnum) and by Servius the Grammarian, the Late Latin author of a commentary on Vergil (modifying or in apposition to the masculine noun equitum).

Citation form: ācer, ācris, ācre

Case	m singular	f singular	n singular	m f plural	n plural
nominative	ācer	ācr·is	ācr·e	ācr·ēs	ācr·ia
genitive	ācr·is			ācr·ium
dative	ācr·ī			ācr·ibus
accusative	ācr·em		ācr·e	ācr·ēs, -īs	ācr·ia
ablative	ācr·ī			ācr·ibus
vocative	ācer	ācr·is	ācr·e	ācr·ēs	ācr·ia

One-termination

The nominative singular form is shared between all three genders, and other forms are built on the oblique stem (found by subtracting -is from the genitive singular). Third declension adjectives of one termination can show various inflection patterns.

Some take the same endings as third-declension adjectives of two or three terminations, with a genitive plural in -ium, neuter nominative/accusative/vocative plural in -ia, an optional i-stem masculine/feminine accusative plural in -īs (alongside consonant-stem -ēs), and normally an i-stem ablative singular in -ī. These endings are used by:

adjectives that end in -āx in the nominative singular and -ācis in the genitive singular (a fairly large category)
Adjectives that end in -ōx, -ōcis or -īx, -īcis: atrōx, ferōx, vēlōx, fēlīx, pernīx (some alternative ablative singular forms, such as īnfēlīce (Catullus 68) and pernīce (Lucretius 2.635) are attested, potentially for metrical reasons).
Multiplicative numerals ending in -plex, -plicis, e.g. simplex, duplex, triplex, multiplex
Adjectives with the form of present participles in -ns, -ntis (see below).
anceps, ancipitis and praeceps, praecipitis

A very small number are attested with consonant-stem endings throughout, such as vetus (although its ablative singular has an alternative form veterī).

Some others are used mostly in the masculine and feminine (sometimes as nouns and sometimes as adjectives) and may show consonant-stem endings in this context, but lack any attested neuter nominative/accusative/vocative plural (at least in Classical Latin). Examples are āles and caelebs. Sometimes a form that is missing in Classical Latin is used or mentioned by a later author.

One-termination, i-stem

Citation form: atrōx, atrōcis

Case	m f singular	n singular	m f plural	n plural
nominative	atrōx		atrōc·ēs	atrōc·ia
genitive	atrōc·is		atrōc·ium
dative	atrōc·ī		atrōc·ibus
accusative	atrōc·em	atrōx	atrōc·ēs, ·īs	atrōc·ia
ablative	atrōc·ī		atrōc·ibus
vocative	atrōx		atrōc·ēs	atrōc·ia

One-termination, consonant stem

Citation form: vetus, veteris

Case	m f singular	n singular	m f plural	n plural
nominative	vetus		veter·ēs	veter·a
genitive	veter·is		veter·um
dative	veter·ī		veter·ibus
accusative	veter·em	vetus	veter·ēs	veter·a
ablative	veter·e		veter·ibus
vocative	vetus		veter·ēs	veter·a

Present participles

Present participles are all declined like one-termination adjectives with a nominative singular in -ns and an oblique stem in -nt-: they inconsistently take either i-stem endings or consonant-stem endings.

For the genitive plural, the i-stem ending -ium is in general more usual. But the consonant-stem ending -um is also found, perhaps especially for participles used as nouns, such as parentum. (The grammarian Servius claims that this ending occurs in the masculine or feminine gender, but should not occur in the neuter, based on the alleged principle that a genitive plural ought not have fewer syllables than the nominative plural.^[3]) In poetry, use of the consonant-stem ending might be motivated by metrical considerations, since a form ending in -ntium could not be used in dactylic verse unless its final heavy syllable was elided into a word-initial light syllable, and this is an uncommon (so-called "harsh") type of elision. The consonant-stem ending has sometimes been printed as -ûm, based on a false hypothesis that it is a contraction of the more usual ending -ium (etymologically, the consonant-stem ending seems instead to be an archaism).
For the neuter nominative/accusative/vocative plural, the i-stem ending -ia is usual. The consonant-stem ending -a is very rare, but is attested in a few cases, such as silenta and fluenta (in place of usual silentia, fluentia).
For the ablative singular, both -e and -ī are used: forms in -e tend to be used in ablative absolute constructions, and more generally, in cases where a participle has verbal force; forms in -ī tend to be used as adjectives, but not exclusively so. Overall, the usage of -e and -ī in the ablative singular shows some degree of instability for many third-declension nouns, adjectives and participles and often does not follow strict or simple rules.

silēns, silentis (“silent”) (present paticiple of *sileō*)
Case	m f singular	n singular	m f plural	n plural
nominative	silēns		silent·ēs	silent·ia (silent·a)
genitive	silent·is		silent·ium (silent·um)
dative	silent·e silent·ī		silent·ibus
accusative	silent·em	silēns	silent·ēs, ·īs	silent·ia (silent·a)
ablative	silent·ī		silent·ibus
vocative	silēns		silent·ēs	silent·ia (silent·a)

Defective adjectives

For some adjectives, certain gender/case/number combinations may be 'defective' (that is, never used, at least in Classical Latin texts).

Some third-declension adjectives of one termination seem to lack a neuter nominative/accusative/vocative plural form. Words of this type often take consonant-stem endings and are typically used either as nouns with masculine or feminine gender, or as adjectives modifying masculine or feminine nouns. Some are attested with adjectival function in combination with neuter nouns, but examples may be limited to cases where the same form is shared between all genders. In this situation, it may be difficult to say whether the word is being used as a true adjective (which by definition shares its gender with the modified noun) or whether it is grammatically a noun in apposition (which may disagree in gender, as in flūmine n Rhodanō m "the River Rhône").

An example is iuvenis:^[4] this can be used as a (usually masculine) noun 'youth, young man' or as an adjective 'young' (typically describing a man, less often a woman): in either case, it takes consonant-stem endings in its oblique forms, such as ablative singular iuvene or genitive plural iuvenum.^[5] It can be categorized as defective in gender because neuter forms, such as a nominative/accusative plural *iuvena, do not occur in Classical Latin. The similar word senex ('old man' rarely 'old woman', or as an adjective 'old, aged') likewise shows consonant-stem inflection and limited use in the neuter gender in Classical Latin: the form senibus, shared between all genders, is attested as a modifier of the neuter noun saeclīs in Ciris (a 1st-century poem from the Appendix Vergiliana), but distinct neuter forms such as a nominative/accusative plural *sena do not occur. (Missing forms of defective adjectives are sometimes created by analogy and used by postclassical authors; for example, in the case of iuvenis, an innovative i-stem neuter plural form iuvenia/juvenia is attested in Medieval Latin and New Latin.)

Words ending in -tor m and -trīx f, usually categorized as agent nouns, may be used adjectivally,^[6] with a similar sense to present participles.

When forms ending in -tor are used this way, the inflection remains the same as that of the noun, with consonant-stem endings. The modified noun is normally grammatically masculine (although not necessarily animate).
- Auctor alone can be used (both as noun and adjective) in the feminine as well as the masculine gender. There are also some examples where a form of auctor is used (either as an adjective or a noun in apposition) to modify a neuter noun, as in "cum ingenio n sibi auctore dignitatem peperissent" (Cicero De Oratore 1.198), "Italiam petiit fatis n auctoribus esto" (Vergil, Aeneid 10.67) and "Italiam si caelo n auctore recusas me pete" (Lucan, Bellum Civile 5.579)
When forms ending in -trīx are used this way, the inflection is mostly the same as that of the noun, but some additional forms may be used following the i-stem inflection patterns of adjectives such as fēlīx. Thus, the word victrīx, when used as an adjective 'victorious', may have an ablative singular either in -e (victrīce) or in -ī (victrīcī). In the singular, such adjectives in -trīx are normally restricted to modifying feminine nouns, but in the plural, they also came to be used sometimes with neuter nouns, taking the i-stem endings -ia in the nominative/accusative/vocative plural and -ium in the genitive plural: thus, phrases such as 'victricia arma' and 'victricium armorum' can be used in Classical Latin.

The Greek endings -ᾰ́ς, -ᾰ́δος f (-ắs, -ắdos) and -ῐς, -ῐδος f (-ĭs, -ĭdos), used to form feminine adjectives from the names of locations or nations, were also used in Latin as endings of feminine adjectives or nouns (either in words taken as a whole from Greek, or in words formed by analogy).^[6]

Comparative (two-termination consonant stem)

Comparative adjectives nearly all follow the same declension pattern, showing a nominative singular in -ior for the masculine and feminine and a nominative singular in -ius for the neuter, with other forms built on a stem in -iōr- combined with consonant-stem endings (such as ablative singular -e, genitive plural -um, and neuter nominative/accusative/vocative plural -a). Alternative i-stem endings are occasionally found for the ablative singular (-ī) and the masculine/feminine accusative plural (-īs). The word plūrēs (plural-only when used as an adjective) shows a modified variant of this declension pattern.

Citation form: melior, melius

Case	m f singular	n singular	m f plural	n plural
nominative	melior	melius	meliōr·ēs	meliōr·a
genitive	meliōr·is		meliōr·um
dative	meliōr·ī		meliōr·ibus
accusative	meliōr·em	melius	meliōr·ēs	meliōr·a
ablative	meliōr·e		meliōr·ibus
vocative	melior	melius	meliōr·ēs	meliōr·a

References

^ 3rd Declension: Locative Case in Meagan Ayer, Allen and Greenough’s New Latin Grammar for Schools and Colleges. Carlisle, Pennsylvania: Dickinson College Commentaries, 2014.
↑ ^2.0 ^2.1 3rd Declension: Summary of i-stem forms in Meagan Ayer, Allen and Greenough’s New Latin Grammar for Schools and Colleges. Carlisle, Pennsylvania: Dickinson College Commentaries, 2014.
^ Servius (c. 4th-5th century A.D.) Charles E. Murgia and Robert A. Kaster, editors, Serviani in Vergili Aeneidos libros IX-XII commentarii, Oxford University Press, published 2018, page 401:
DEFENDENTUM ideo defendentum et ruentum quia ad masculinum retulit et 'hi defendentes' facit, quod et de feminino dicimus: si autem de neutro loquamur, 'defendentium' dicamus necesse est, quia 'haec defendentia' facit et, ut supra diximus, genetivus pluralis a nominativo plurali minor esse non debet.
^ Irregularities and Special Uses of Adjectives in Meagan Ayer, Allen and Greenough’s New Latin Grammar for Schools and Colleges. Carlisle, Pennsylvania: Dickinson College Commentaries, 2014.
^ 3rd Declension: Summary of I-stem Forms in Meagan Ayer, Allen and Greenough’s New Latin Grammar for Schools and Colleges. Carlisle, Pennsylvania: Dickinson College Commentaries, 2014.
↑ ^6.0 ^6.1 Madvig, I. N. (1856) George Woods, transl., A Latin Grammar for the Use of Schools, 3rd edition, pages 61-63