4.7.1. The Predeclensional vowel is that which precedes the ending, even the Ø ending; i.e., we say that pater has a long predeclensional vowel; that the Vocative pater has a full one, and that medhu has it Ø. Examples of the three are pod-s, pod-os or owi-os.
NOTE. The vocalic changes in timbre and grade aren't meaningful by themselves, they are multifunctional: they can only have meaning in a specific declension, and it is not necessarily always the same. They are thus disambiguating elements, which help distinguish homophones (i.e., words that sound alike).
4.7.2. Two kinds of nominal inflection have no alternating vowel: that in -i or -u, and that of the reduplicate participles.
4.7.3. Stems in -r and -n have two possibilities, both with a Nominative singular in Ø and lengthened vowel.
1. Nom. with lengthened vowel, Acc. and Voc. with full vowel, and Gen. Ø. The timbre can be e or o, depending on the words.
a. In -r, as in Nom. mater, Acc. materm [ma:ter'm], Voc. mater, Gen. matros.
b. In -n, in root stems, as in dog: Nom. kwon/kuwon, Acc. kwonm [kwon'm] / kuwonm [kuwon'm], Voc. kuon/kuwon, Gen. kunos.
2. Sometimes, the Genitive has a full grade as the Accusative and the Vocative. This grade is redundant, not necessary for the disambiguation of the Genitive from the Nominative. There are, as above, different timbres e and o, sometimes o in Nom.-Acc.-Voc and e in Gen., sometimes o in Acc.-Voc.-Gen. and e in Obl.
4.7.4. There is usually the same vocalism in nouns ending in occlusive.
An exception is that of the adjectives and participles in -nt, which presents long vowel in the Nominative, full vowel in Accusative and Vocative, and Ø in the Genitive: bheront/bherontos or bherent/bherentos
NOTE. There are remains of what seems to be an older alternating vocalism in monosyllabics. The variants of ped/pod suggest that it comes from a paradigm Nom. pod-s, Acc. pod-m [pod'm], Gen. ped-os. This is, again, mostly irrelevant for modern Europaio, in which both alternating forms are given.
4.7.5. Stems in -s do not present a Ø-grade. Animates, as already said, oppose a lengthened vowel Nominative to the other cases, which have full vowel; as in -es / -es, -os / -os
4.7.6. We know already what happens with stems in -i, -u, which have two general models:
1. Nom. -i-s, Acc. -i-m, Voc. -ei or -i, Gen. -i-os / Nom. -u-s, Acc. -u-m, Voc. -ei or -i, Gen. -u-os
2. Nom. -i-s, Acc. -i-m, Voc. -eu or -u, Gen. -ei-s / Nom. -u-s, Acc. -u-m, Voc. -eu or -u, Gen. -eu-s
NOTE. This is an inversion of the normal situation: the Nom.-Acc.-Voc. has Ø grade (but for some Voc.), the Gen. Ø or full. Distinction is obtained through alternating forms; as in Voc., in which the ending -ei distinguishes it from Neuters in -i; or with changes of e/o.
4.7.7. Those in long diphthong alternate the diphthong (or long vowel) with j or w, which represent the Ø grade; as in djeus, djem, diwos. Uniform vocalism (i.e., no vowel change) is generalized, though; as in naus, naw-os.
NOTE. These diphthongs reflect a possible older situation of a vowel plus a laryngeal (as *-eh) and they are probably related to nouns in -a and also to those in -e and -o. It is a matter of speculation, though, as we can only know with some certainty the IE III.
4.7.8. Stems in-a usually maintain an uniform vocalism: Nom.-Voc. -a, Acc. -am, Gen. -as. But those in -ia/-i/-ia can alternate Nom.-Voc. -ia/-i / Gen. -ias.
There are also remains of -a in Voc. (and even Nom.), as well as -ai, cf. gr. gúnai (and derived forms in -ai in Old Indian and other dialects). The -e and -o endings have also traces of alternating phonetic changes.
NOTE. In gr. Europe / Europa, the Genitive is Europ-ai-os, which gives also the adjective (and hence the Europaio nominalized form Europaios/m). In lat. this -ai-o- is -ae-u-, and so Europae-us, -a, -um. See § 1.7.5.
4.7.9. Finally, the neuter stems distinguish the Nom.-Acc.-Voc. forms by having a predeclensional vowel normally Ø (the ending is also Ø, but for the thematic), as we have seen in nouns ending in -i, -u, -r, -n and occlusive; like in madhu, nomn, krd. There are exceptions, though:
1. Nouns with lengthened or full vowel; as, gr. udor and oi. ahar.
2. Nouns in -s cannot have Ø, they have o in nouns, e in adjectives; as in genos, race; eugenes, of good race.
3. Finals -e/o have a uniform predeclensional vowel, normally o, plus the ending -m of Nom.-Acc.-Voc.
NOTE 1. In the Oblique cases, neuters are declined like the animates.
NOTE 2. There are no neuters in -a, but for those which became plural.