Grammaticalization is the subjection of a linguistic construction or construction schema or of a linguistic operation to rules of grammar. The most straightforward examples of the process concern words which start out as lexical items but are used in a grammatical function and thus become grammatical formatives.

The English preposition of is such a case. Proto-Germanic had inherited from Proto-Indo-European the word *ab. It functions as a preposition meaning ‘from’ and as an adverb meaning ‘away’. In both of these functions, it appears in Old English in the variants of/af/æf. illustrates its use as a preposition.

In Middle English, only the phonological variant of survives, still conserving both functions. In its function as an adverb, the word is stressed and spelled off from 1200 on. In Modern English, this is also the spelling of the preposition in its concrete local use ‘away from’, as in .

Old English marked the genitive exclusively by declension of the dependent noun ().

In Middle English, of is increasingly used to mark the genitive relation, as in the translation of and in .

At the same time, French already has expressions like .

This also uses an erstwhile ablative preposition as a genitive marker. The development of English of did not happen independently of contact with the French case.

In contemporary English, of is a genitive marker beside the so-called Saxonian genitive marked by 's. Thus a and b are synonymous.

At this point, of has become a case marker, thus an exponent of grammatical structure. We thus have a change from a relatively free construction with relatively concrete meaning involving lexical items to a relatively tight grammatical construction where the former lexical item only fulfills a structural function. This directed variation is grammaticalization. It is at work in all parts of grammar and omnipresent in all languages at all times. It is the major process creating grammatical structure (Heine & Kuteva 2007, Smith 2011, Lehmann 2024).

In the same way, morphology develops on the basis of syntax. Vulgar Latin had a periphrastic construction combining the infinitive of the full verb with a finite form of the verb ‘have’, illustrated by . The direct object could precede either the auxiliary, as in #a, or the infinitive, as in #b.

Grammaticalization reduces all three word forms composing the construction, so it takes the form of . The order variants persist in Ibero-Romance.

Continuing grammaticalization converts the periphrastic construction into a complex verb form with future meaning. The Portuguese form in continues variant #a, while the Spanish form in continues variant #b.

This is, again, a historical case of grammatical change. However, it illustrates at the same time the way that morphology appears first in human language.