In combination with classification by gender, Adûnaic nouns can be classified as either strong-nouns or weak-nouns (SD/425). Strong nouns modify the vowel of their last syllable when declined into most noun cases, while weak nouns add some sort of suffix (SD/430). Generally speaking, monosyllabic nouns and nouns with a long vowel or diphthong in their last syllable are classified as weak (SD/431), while polysyllabic nouns with a short vowel in their last syllable are strong.

In addition to the strong and weak classification, Adûnaic noun declensions differ for nouns ending in a consonant (Class I) versus those ending in a vowel (Class II). For Class I nouns, there is at most one final consonant, since clusters are not allowed as final-consonants in Adûnaic. In combination with the strong and weak classifications, this gives us four groups:

Classes of Adûnaic Nouns
Strong I	Ending in a single consonant preceded by a short vowel	✧ SD/430, 436-7
Strong II	Ending in a short vowel	✧ SD/431, 437
Weak I	Ending in a single consonant preceded by a long vowel or diphthong	✧ SD/431-2, 437-8
Weak II	Ending in a long vowel or diphthong	✧ SD/438

Tolkien labeled the weak classes Weak (a) and Weak (b) (SD/437-8), but the discussion here uses Weak I and Weak II for consistency with the Strong class names. Also remember that monosyllables are always weak, though all attested Adûnaic monosyllables have a long vowel or diphthong making them weak in any case.

There is a small group of nouns (Strong Ib and Strong IIb) that used strong declensions despite having a long vowel in their last syllable (SD/437). These declensions are archaic, however, and the weak declensions can be used for such nouns by the time of Classical Adûnaic.

Noun declensions also vary depending on the gender of the noun. These three factors combine to determine how a noun is declined: gender, strong versus weak, class I versus class II.