In Ancient Greek grammar, privative a (also known as privative alpha; in Latin, α prīvātīvum, in Greek, α στερητικόν) is the prefix a- that expresses negation or absence (e.g. a-theos, ). It is derived from a Proto-Indo-European syllabic nasal *, the zero ablaut grade of the negation *ne, i.e. /n/ used as a vowel. For this reason, it usually appears as an- before vowels (e.g. an-alphabetism, an-esthesia, an-archy). It shares the same root with the Greek prefix nē or ne, in Greek νη or νε, that is also privative (e.g. ne-penthe).