The notion of valency describes the property of verbs to open argument positions in a sentence (e.g., the verb eat opens two argument positions, filled in the sentence John ate the cake by the subject John and the direct object the cake). Depending on the number of arguments, a verb is avalent (no argument), monovalent (one argument), bivalent (two arguments), or trivalent (three arguments). In Romance languages, verbs are often labile (i.e., they occur in more than one valency pattern without any formal change on the verb). For example, the (European and Brazilian) Portuguese verb adoecer ‘get sick’/‘make sick’ can be used both as a monovalent and a bivalent verb (O bebê adoeceu ‘The baby got sick’ vs. O tempo frio adoeceu o bebê ‘The cold weather made the baby sick’). However, labile verbs are not equally important in all Romance languages. Taking the causative–anticausative alternation as an example, labile verbs are used more frequently in the encoding of the alternation in Portuguese and Italian than in Catalan and Spanish (the latter languages frequently recur to an encoding with a reflexively marked anticausative verb (e.g., Spanish romperse ‘break’). Romance languages possess various formal means to signal that a given constituent is an argument: word order, flagging the argument (by means of morphological case and, more importantly, prepositional marking), and indexing the argument on the verb (by means of morphological agreement or clitic pronouns). Again, Romance languages show variation with respect to the use of these formal means. For example, prepositional marking is much more frequent than morphological case marking on nouns (the latter being only found in Romanian).
The Causative/Inchoative alternation involves pairs of verbs, one of which is causative and the other non-causative syntactically and semantically (e.g., John broke the window vs. The window broke). In its causative use, an alternating verb is used transitively and understood as externally caused. When used non-causatively, the verb is intransitive and interpreted as spontaneous. The alternation typically exhibits an affected argument (i.e., a Theme) in both intransitive and transitive uses, whereas the transitive use also involves a Causer that brings about the event. Although they are often volitional agents (e.g., John broke the window with a stone), external causers may also be non-volitional causers (e.g., The earthquake broke the windows) and instruments (e.g., The hammer broke the window). Morphologically, languages exhibit different patterns reflecting the alternation, even intralinguistically. In languages like English, alternations are not morphologically coded, but they are in most languages. Languages like Hindi commonly mark causative (or transitive) alternations by means of different mechanisms, such as internal vowel changes or causative morphology. In many European languages, a subset of alternating verbs may exhibit an uncoded alternation, but most alternating verbs mark anticausativization with a reflexive-like clitic. In Yaqui (Uto-Aztecan), different patterns are associated with different verbal roots. The alternation may be uncoded, equipollent (i.e., both alternating forms are coded), and anticausative. Theoretically, different approaches have explored the alternation. Both lexical and syntactic causativization and anticausativization accounts have been proposed to explain the alternation. A third approach postulates that both forms are derived from a common source.