Tertia Dēclīnātiō II

Many nouns of the third declension show traces of an old stem in -i-. These are called i-stems. Most of them have become assimilated to consonant stems and can be distinguished only by the genitive plural in -ium (sometimes the accusative singular in -im or the ablative singular in -ī).

The i-stem is therefore originally distinct from the consonant stem, but in Latin the two have become so confused that for practical purposes the third declension must be treated as a single declension with two parallel sets of endings.

Pure i-stems are recognized by the genitive plural in -ium and (in many) by the accusative singular in -im and the ablative singular in -ī. Masculine and feminine i-stems end in the nominative singular in -is or -ēs; neuter i-stems end in -e, -al, or -ar.

Examples: turris, -is (f.) tower; ignis, -is (m.) fire; sitis, -is (f.) thirst; sedīle, -is (n.) seat; animal, -ālis (n.) animal; calcar, -āris (n.) spur.

The accusative singular in -im is regularly found in: (a) Greek nouns (e.g. basis, basim); (b) names of rivers and towns ending in -is (Tiberis, Tiberim); (c) tussis, sitis, vīs; and (d) frequently in turris, secūris, puppis, febris, restis.

The ablative singular in -ī appears in the same words and also in adjectives of one ending used as nouns (amantī, with the lover) and in many neuters in -e, -al, -ar.

The mixed i-stems are nouns originally consonantal that have adopted some i-stem features. They include: (1) nouns in -ēs with genitive in -is (nūbēs, nūbis, f., cloud); (2) monosyllables ending in two consonants (urbs, urbis, f., city; nox, noctis, f., night; ars, artis, f., art); (3) some others such as cliēns, cliēntis (m./f., client) and aetās, aetātis (f., age).

These nouns regularly form genitive plural in -ium, but otherwise inflect like consonant stems.

A few nouns vacillate between i-stem and consonant inflection. cliēns (m.), cliēntis, takes gen. pl. either cliēntum or cliēntium. Likewise parēns, parentis shows both parentum and parentium.

The kinship words pater, māter, frāter, however, treat the family relationship as a fixed unit and keep the older consonant inflection: gen. pl. patrum, mātrum, frātrum — never -ium. iuvenis and senex likewise resist the i-stem ending.

General gender rules of the third declension:

Masculine are nouns ending in -or, -ōs, -er, -ex, -es (gen. -itis): victor, flōs, pater, rēx, mīles.

Feminine are nouns ending in -ās, -ēs, -is, -aus, -s (after a consonant), -x, -dō, -gō, -iō: aetās, nūbēs, turris, laus, urbs, lex, virtūs (for -tūs), imāgō, virgō, nātiō.

Neuter are nouns ending in -a, -e, -ar, -al, -en, -ur, -us, -c, -t, -l: poēma, sedīle, calcar, animal, nōmen, augur, genus, lac, caput, mel.

Important exceptions: sōl (m.), sāl (m./n.), arbor (f.), cor (n.), iter (n.), ōs (n., mouth) and os (n., bone), vās (n., vessel) vs vas (m., surety).

A small group of nouns is so anomalous that each must be learned separately. The most important are vīs (force), sūs (swine), bōs (ox), Iuppiter (Jupiter), senex (old man), iter (journey), and caput (head).

In vīs, the singular is from one stem (vī-), the plural from another (vīr-). Iuppiter is a frozen vocative compound (Iuppiter < Iov-pater); the rest of the cases use the stem Iov-: Iovis, Iovī, Iovem, Iove.

Iter is neuter; gen. itineris, dat. itinerī, etc. The stem alternates iter-/itiner-.