Proto-Germanic nouns are declined according to number (singular and plural) and case (nominative, vocative, accusative, genitive, dative and instrumental). Furthermore, each noun has an assigned gender, which determines the inflection of that noun but also of any pronouns or adjectives that modify or refer to that noun. There are three genders: masculine, feminine and neuter. Often but not always, the gender can be determined from the nominative singular form of the noun. The nominative and vocative plural forms are always alike, and neuter nouns have only a single form for the nominative, vocative and accusative cases.
Nouns can be divided into several declension classes based on the formation of the case-and-number endings. Globally, there are vowel stems (a-, ō-, i- and u-stems) and consonant stems (n-, r- and z-stems and stems ending in other consonants). Usually, only nouns ending in consonants other than n, r or z are called consonant stems.
The a-stems are the most common type of noun in Proto-Germanic, and can be either masculine (ending in -az) or neuter (ending in -ą, nasal a). The two genders differ only in the nominative, vocative and accusative cases; the other three cases are identical for both. There are two smaller subgroups within the a-stems: ja-stems and wa-stems. These are declined mostly the same as regular a-stems, however.
The ō-stems are also common, and are always feminine, with the nominative singular ending in -ō. They are the feminine equivalent of the a-stems. There are also jō-stems and wō-stems, declined identical to the regular ō-stems.
The ī/jō stems are a small group of feminine nouns that inflect mostly as jō-stems, but which have a nominative and vocative singular ending in -ī instead of -jō.
The i-stems are reasonably common, and appear in all three genders, although neuter i-stems are very rare. The masculine and feminine i-stems are declined the same, with a nominative singular in -iz. The neuters end in -i.
The u-stems are fairly common, and are mostly analogous to i-stems. The masculine and feminine are identical, ending in -uz. Neuters are very rare, and end in -u.
The an-stems are a common group of noun and are either masculine or neuter, although neuters are rare. Their nominative singular forms end in -ô (overlong). There are also jan-stems and wan-stems, which are declined mostly as regular an-stems.
The ōn-stems are also common and are always feminine, ending in -ǭ (long nasal o). They are the feminine counterpart of the an-stems, just like ō-stems are the feminine counterpart of a-stems. There are also jōn-stems and wōn-stems, declined identical to regular ōn-stems.
This group of nouns contains only a single type of abstract noun, formed by attaching an īn-suffix to adjectives. They are always feminine, and are essentially identical to ōn-stems, with ī replacing ō in all the forms. The nominative singular ends in -į̄ (long nasal i).
The z-stems are fairly rare, and are always neuter. They are formed similar to an-stems, but with z replacing n. Their nominative singular forms end in -az, like masculine a-stems.
Other consonant stems
Nouns in this group are usually just called 'consonant stem'. It is mostly a class of remnants, consisting of root nouns (nouns with no suffix) and nouns with a suffix ending in a consonant other than n, r or z. They can be any gender, and masculine and feminine are identical, with nominative singular forms ending in -s or -z based on the voicing of the previous consonant. There are few reconstructible neuters; those that can be reconstructed have no ending in the nominative singular. It is possible that many a-stem neuters actually or originally belonged to this class, however.