linguistic:voice_change, information:highlighting
Strong vs. weak nouns and adjectives

Originally, adjectives in Proto-Indo-European followed the same declensional classes as nouns. The most common class (the o/ā class) used a combination of o -stem endings for masculine and neuter genders and ā -stems ending for feminine genders, but other common classes (e.g. the i class and u class) used endings from a single vowel-stem declension for all genders, and various other classes existed that were based on other declensions. A quite different set of "pronominal" endings was used for pronouns, determiners, and words with related semantics (e.g., "all", "only").

An important innovation in Proto-Germanic was the development of two separate sets of adjective endings, originally corresponding to a distinction between indefinite semantics ("a man") and definite semantics ("the man"). The endings of indefinite adjectives were derived from a combination of pronominal endings with one of the common vowel-stem adjective declensions – usually the o/ā class (often termed the a/ō class in the specific context of the Germanic languages) but sometimes the i or u classes. Definite adjectives, however, had endings based on n -stem nouns. Originally both types of adjectives could be used by themselves, but already by Proto-Germanic times a pattern evolved whereby definite adjectives had to be accompanied by a determiner with definite semantics (e.g., a definite article, demonstrative pronoun, possessive pronoun, or the like), while indefinite adjectives were used in other circumstances (either accompanied by a word with indefinite semantics such as "a", "one", or "some" or unaccompanied).

In the 19th century, the two types of adjectives – indefinite and definite – were respectively termed "strong" and "weak", names which are still commonly used. These names were based on the appearance of the two sets of endings in modern German. In German, the distinctive case endings formerly present on nouns have largely disappeared, with the result that the load of distinguishing one case from another is almost entirely carried by determiners and adjectives. Furthermore, due to regular sound change, the various definite (n -stem) adjective endings coalesced to the point where only two endings (-e and -en) remain in modern German to express the sixteen possible inflectional categories of the language (masculine/feminine/neuter/plural crossed with nominative/accusative/dative/genitive – modern German merges all genders in the plural). The indefinite (a/ō -stem) adjective endings were less affected by sound change, with six endings remaining (-, -e, -es, -er, -em, -en), cleverly distributed in a way that is capable of expressing the various inflectional categories without too much ambiguity. As a result, the definite endings were thought of as too "weak" to carry inflectional meaning and in need of "strengthening" by the presence of an accompanying determiner, while the indefinite endings were viewed as "strong" enough to indicate the inflectional categories even when standing alone. (This view is enhanced by the fact that modern German largely uses weak-ending adjectives when accompanying an indefinite article, and hence the indefinite/definite distinction no longer clearly applies.) By analogy, the terms "strong" and "weak" were extended to the corresponding noun classes, with a -stem and ō -stem nouns termed "strong" and n -stem nouns termed "weak".

However, in Proto-Germanic – and still in Gothic, the most conservative Germanic language – the terms "strong" and "weak" are not clearly appropriate. For one thing, there were a large number of noun declensions. The a -stem, ō -stem, and n -stem declensions were the most common and represented targets into which the other declensions were eventually absorbed, but this process occurred only gradually. Originally the n -stem declension was not a single declension but a set of separate declensions (e.g., -an, -ōn, -īn) with related endings, and these endings were in no way any "weaker" than the endings of any other declensions. (For example, among the eight possible inflectional categories of a noun — singular/plural crossed with nominative/accusative/dative/genitive — masculine an -stem nouns in Gothic include seven endings, and feminine ōn -stem nouns include six endings, meaning there is very little ambiguity of "weakness" in these endings and in fact much less than in the German "strong" endings.) Although it is possible to group the various noun declensions into three basic categories — vowel-stem, n -stem, and other-consonant-stem (a.k.a. "minor declensions") — the vowel-stem nouns do not display any sort of unity in their endings that supports grouping them together with each other but separate from the n -stem endings.

It is only in later languages that the binary distinction between "strong" and "weak" nouns become more relevant. In Old English, the n -stem nouns form a single, clear class, but the masculine a -stem and feminine ō -stem nouns have little in common with each other, and neither has much similarity to the small class of u -stem nouns. Similarly, in Old Norse, the masculine a -stem and feminine ō -stem nouns have little in common with each other, and the continuations of the masculine an -stem and feminine ōn/īn -stem nouns are also quite distinct. It is only in Middle Dutch and modern German that the various vowel-stem nouns have merged to the point that a binary strong/weak distinction clearly applies.

As a result, newer grammatical descriptions of the Germanic languages often avoid the terms "strong" and "weak" except in conjunction with German itself, preferring instead to use the terms "indefinite" and "definite" for adjectives and to distinguish nouns by their actual stem class.

In English, both sets of adjective endings were lost entirely in the late Middle English period.