Languages code aspects of human cognition and communication in their system. Certain aspects are recurrently coded in their grammar, in the form of grammatical categories, relations and operations and of the various formal processes in which these are manifested. The total of the cognitive and communicative functions underlying the semantics of languages can be systematized in terms of a set of functional domains. They are set up in such a way as to accommodate the grammatical phenomena known from the languages of the world. Every language allows its speakers to code the basic concepts and operations of each of these domains. However, some do it by lexical-syntactic strategies, others by grammatical, including morphological, strategies. A large part of what is conveyed in communication is not coded at all, but left to inference.

Linguistic change and variation are essential aspects of human languages. Different kinds of them, including phonological, grammatical and semantic change and variation, as such are universal. Grammatical change and variation comprises several more specific kinds, including analogy, reanalysis and grammaticalization. Grammaticalization occurs in all languages. In principle, no part of grammar is exempt from it, although a language may take some tens of thousands of years until it grammaticalizes a certain functional category.

In this perspective, grammaticalization is the transfer of a functional category and associated operations into grammar by subjecting them to rules of the system. Few languages grammaticalize functional concepts and operations in all functional domains. All languages grammaticalize them in at least some domains. Moreover, as broad as these domains are presently conceived, some languages grammaticalize more than one subdomain of a given domain. As a result, the morphology of some languages is more complex than the average.

The same set of functional domains underlies the grammar of all languages. Most aspects of human experience, including, as random examples, cooking terminology or interaction with supernatural beings, are outside these domains and are therefore never grammaticalized. The reverse side of this coin is that certain grammatical categories and strategies, if conceived at the typological level, are shared among many languages. The processes of reducing the underlying functional concepts and operations to linguistic structure converge on recurrent patterns.

The ideal complete process of grammaticalization can be divided into two phases. The initial phase consists in the recruition of a lexico-syntactic construction for grammaticalization; the concluding phase consists in its complete integration into the morphology and its reduction to zero. In the initial phase, speakers freely activate their expressive imagination. In the second phase, increasing automation in the use of the grammaticalized construction leads to the all but irreversibility of the process and to the ultimate semantic and phonological erosion of the language signs involved. The term ‘degrammaticalization’ designates a process running in the opposite direction of grammaticalization. Few genuine cases of degrammaticalization have been historically documented. They presuppose an exceptional degree of creativity and non-conformism in the speakers initiating them.

Grammaticalization of linguistic signs and the automation in their processing increase the redundancy in messages and relieve interlocutors from the task of inferencing. This is the main raison d'être of grammar and of grammaticalization.