Among intransitive verbs, a small subset is able to form a non-elliptical, grammatical clause without a single argument. These verbs are described as having a valency of 0, while other intransitive verbs, which require a single argument, have a valency of 1. Avalent verbs, or verbs with null valency, form a limited, closed class within intransitive verbs. Not all languages contain this class.
ZeroIntr: Avalent verbs, characterized by being able to form a non-elliptical, grammatical clause without a single argument, exist as a subclass of intransitive verbs.
NoZeroIntr: All intransitive verbs have a required valency of 1.
 It is important to distinguish avalent verbs from those verbs, present in many languages, that can form sentences alone but nevertheless fail to constitute non-elliptical constructions, since the otherwise explicit pronoun or noun is expressed on the verb itself (also known as pro-drop). Such verbs may even be transitive, as in the Hungarian szeretlek (‘I love you’). Impersonal verbs that require an argument are also excluded from the avalent category, such as the Latin taedet me ‘it disgusts me,’ in which the object me is in the accusative case.
 The Hungarian esteledik (‘night falls’), which can stand alone as a complete, grammatically correct sentence, is one example of an avalent verb. While it can occur with adjuncts (for example, Nyáron később esteledik, mint télen ‘Night falls later in summer than in winter’), the presence of these adjuncts is not grammatically essential.