4.1.1. Declension is made by adding terminations to different stem endings, vowel or consonant. The various phonetic changes in the language have given rise to the different declensions. Most of the case-endings, as shown in this Modern Indo-European grammar, contain also the final letter of the stem.

Adjectives are generally declined like nouns, and are etymologically to be classed with them, but they have some peculiarities of inflection which will be later explained.

4.1.2. Nouns and adjectives are inflected in four regular Declensions, distinguished by their final phonemes – characteristic of the Stem –, and by the opposition of different forms in irregular nouns. They are numbered following Graeco-Latin tradition: First or a-Declension, Second or o-Declension, Third or i/u-Declension, Fourth or Consonant Declension, and the variable nouns.

NOTE. The Second or o-Declension is also the Thematic Declension, opposed to the rest – and probably older in the evolution of PIE nominal inflection –, which form together the Athematic Declension.

The Stem of a noun may be found, if a consonant stem, by omitting the case-ending; if a vowel stem, by substituting for the case-ending the characteristic vowel.

NOTE. Most Indo-Europeanists tend to distinguish at least two major types of declension for the oldest PIE, Thematic and Athematic. Thematic nominal stems are formed with a suffix -o- (in vocative -e), and the stem does not undergo ablaut. The Athematic stems are more archaic, and they are classified further by their ablaut behaviour: acro-dynamic, protero-dynamic, hystero-dynamic and holo-dynamic, after the positioning of the early PIE accent in the paradigm. For more on this, see Beekes (1995) and Meier-Brügger (2003).

4.1.3. The following are General Rules of Declension:

a. The Nominative singular for animates ends in -s when the stem endings are i, u, ī, ū, Diphthong, Occlusive and Thematic (-os), or -Ø in ā, Sonant and s; while in the plural -es is general, -s for those in ā, and -os for the Thematic ones.

b. The Accusative singular of all masculines and feminines ends in -m or -ṃ (after consonant), the Accusative plural in -ns or -ṇs.

c. The Vocative singular for animates is always -Ø, and in the plural it is identical to the Nominative.

d. The Genitive singular is common to animates and inanimates, it is formed with -s: -s, -es, -os. A very old alternative possibility is extended -os-jo. The Genitive plural is formed in -ōm, and in -ām in a-stems.

e. The Obliques singular end usually in -i: it can be -i, -ei, -ēi, -oi, -ōi or -āi, and their extensions. In the plural, there are two series of declensions, Instr. -bhis/-mis (from Sg. -bhi), Dat.-Abl. -bhos/-mos (PII -bhjas) as well as (BSl. and PII) Loc. in -su, Gk. -si.

NOTE. Meier-Brügger (2003) considers that “[e]vidence seems to indicate that while the dative and ablative plural were marked with *-mos, the instrumental plural was marked with *-b^hi” in PIH, and similarly Mallory & Adams (2006) differentiate for the oldest PIE declension a Dat. -mus, instrumental -bhi, and Abl. -bh(j)os. Comparison shows an Ins. Sg. -bhi, (cf. Gk. -p^hi, Myc -pi, and also Arm. Ins. marb), BSl. -mi (cf. Lith. akmenimì, O.C.S. kamenĭmĭ) and for North-West IE dialects a division between Italic+Celtic and Germanic+Balto-Slavic Plural forms: Celtic shows traces of an Instrumental -bhis (cf. O.Ir. Dat.-Loc.-Inst.-Abl. cridib, and in Graeco-Aryan O.Ind. sūnúbhis, Av. bāzubīs, Arm. srtiwkh), Italic and Celtic show a Dat.-Abl. -bhos (cf. Celtiberian Dat.-Loc.-Inst.-Abl. arecoraticubos, Lat. matribus, Osc. luisarifs), while Balto-Slavic shows Inst. -mis (cf. Lith. sunumìs, O.C.S. synumĭ), Dat.-Abl. -mos (cf. O.C.S. synŭmŭ, Lith. sūnùms, sūnùmus), and Germanic shows a Dat.-Abl.-Inst. -m-. Also, Indo-Iranian -bhjas (<*-bhjos), according to Meier-Brügger, “can thus be regarded as a cross between the instrumental *-b^hi  and the dative/ablative *-mos”. Even if some might consider these data enough to draw conclusions about a well-differentiated common PIH plural declension system, we think it is more appropriate to maintain in MIE the (conservative) reconstructible North-West IE West/East dialectal differentiation, i.e. Dat.-Abl. -bhos/-mos (PII -bhjas), and Instr. -bhis/-mis, without using any of those assimilation theories proposed, as e.g. PIE Inst. *-m- → dialectal *-bh-.

f. Inanimates have a syncretic form for Nom.-Ac.-Voc. -Ø in Athematic, or -m in Thematic. The plural forms end in -ā in thematics and -a in athematics.

NOTE. About the nominative/accusative neuter plural, Meier-Brügger states: “in terms of content, the idea of a collective mass is certainly dominant. Therefore, the collective suffix (= athematic *-h₂- and thematic *-e-h₂-) is used, no ending (zero) added (…) The understanding of the neuter plural as collective explains the ancient IE characteristic, observable in isolated cases, of combination of the neuter plural and the singular of a verb (…)”.

g. All Animates have the same form in the plural for Nom.-Voc., in -es.

4.1.4. The so-called Oblique cases – opposed to the Straight ones, Nom.-Acc.-Voc –, are Genitive and the Obliques proper, i.e. Dative, Locative, Instrumental and Ablative. IE languages show an irregular Oblique declension system.

NOTE. Sanskrit or Avestan had 8 cases, Anatolian and Italic dialects show up to 8 (cf. Osc. Loc. aasai for Lat. ‘in ārā’, or Ins. cadeis amnud for Lat. ‘inimicitiae causae’, preiuatud for Lat. ‘prīuātō’, etc.), while Latin shows six and a semisystematic Locative notion; Balto-Slavic shows seven, Mycenaean at least six cases, while Koiné Greek and Proto-Germanic had five.

Nominal Desinences (Summary)