Although it might seem so, there is not an insurmountable barrier between the lexicon and grammar of a language. Consider that a lexicon has a grammatical organization of its elements, which in fact are divided into nouns, adjectives, verbs, adverbs, etc. In addition, there are many types of Italian nouns (such as proper nouns, common nouns, collective nouns, abstract nouns, and concrete nouns), which is also the case with other parts of speech.

Also, lexical elements can be used for grammatical purposes, and, conversely, grammatical elements can be used to create lexical elements. At the same time, the relationship between words can be expressed via grammatical means or by a combination of words. Consider these two sentences:

L'azienda è in crisi per le dimissioni del direttore.
L'azienda è in crisi a causa delle dimissioni del direttore.

In the second sentence, the combination a causa di, consisting of "preposition + noun + preposition," is equivalent to the preposition per in the first sentence; a causa di is a locuzione preposizionale (prepositional phrase) in which a lexical item (i.e., a noun) is used for a grammatical end: it helps to replace a preposition. The same phenomenon occurs with other prepositional phrases: ad opera di, da parte di have a value similar to that of the preposition da; a seguito di may substitute per or dopo; in mezzo a corresponds to tra; per mezzo di can replace con.

A phrase composed of "verb + direct object" can also replace a simple verb; for example: far uso = usare, dare congedo = congedare, prendere la fuga = fuggire.

In this instance a grammatical construction replaces a word.

These examples demonstrate that there is a sort of interchange between the domain of lexicon and grammar. This phenomenon is clearly seen over time by observing the evolution of the language.

• The term lessicalizzazione (lexicalization) refers to the process by which a set of elements governed by grammatical relationships becomes a unit, something that amounts to a single word.

For example, d'un tratto (all of a sudden), ora come ora (right now) are phrases that are equivalent to the unique words improvvisamente (suddenly), momentaneamente (momentarily); similarly senza capo né coda equals inconcludente (inconclusive).

Another type of lexicalization occurs when a grammatical form becomes a vocabulary unit: for example, the present participle cantante, the gerund reverendo, or the infinitive piacere.

• Then there is the opposite process: grammaticalizzazione (grammaticalization). A word becomes, in the course of linguistic evolution, a grammatical affix or function word.

The extant preposition mediante was once the present participle of the verb mediare; l'amico mediante (that is, "being a friend as mediator") has become mediante l'amico; mediante then took the form of a preposition: mediante le promesse, mediante gli aiuti. The same phenomenon occurred with durante and nonostante, previously the present participles of durare and ostare, and tranne, formerly the imperative of trarre.

The phenomenon of grammaticalization is common to all languages: it is recognized that various prepositions and suffixes are derived from obsolete word forms, providing a full semantic content. From the domain of the lexicon they pass to the grammatical.