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In historical linguistics, betacism is a sound change in which [b] (the voiced bilabial plosive, like in bane) shifts to [v] (the voiced labiodental fricative, like in English vane). It is an example of lenition. Betacism is a fairly common phenomenon; it has taken place in Greek, Hebrew, Portuguese and Spanish, among others. In Classical Greek, the letter beta <β> denoted [b]. As a result of betacism, it has come to denote [v] in Modern Greek, a process which probably began during the Koine Greek period, approximately in the 1st century AD, along with the spirantization of the other δ and γ. Modern (and earlier Medieval) Greek uses the digraph <μπ> to represent [b]. Indeed, this is the origin of the word betacism.
Perhaps the best known example of betacism is in the Romance languages. The first traces of betacism in Latin can be found in the third century C.E. The results of the shift are most widespread in the Western Romance languages, especially in Spanish, where the letters <b> and <v> are now both pronounced [β] (the voiced bilabial fricative, which is similar to [v]) except phrase-initially and after [m] when they are pronounced [b]; the two sounds ([β] and [b]) are now allophones. A similar phenomenon takes place in Persian in casual speech. Another example is in Neapolitan, or in Maceratese (dialect of Macerata) which uses <v> to denote betacism-produced [v], such that Latin bucca corresponds to Neapolitan vocca and to Maceratese "vocca", Latin arborem to arvero or arvulo, and barba to Neapolitan varva and Maceratese "varba".
Betacism occurred in Ancient Hebrew; the sound [b] (denoted <ב>) changed to [β] and eventually to [v] except when geminated or when following a consonant or pause. As a result, the two sounds became allophones; but, due to later sound changes, including the loss of gemination, the distinction became phonemic in Modern Hebrew.
- An intermediate value of [β] is likely. Evidence for this sound change includes use of the letter β to transcribe Latin v and interchanges with the αυ/ευ diphthongs which had developed fricative pronunciations, c.f. Gignac, Francis T. (1970). "The Pronunciation of Greek Stops in the Papyri". Transactions and Proceedings of the American Philological Association: 188.
- The use of μπ, vτ, γκ for voiced plosives is related to another development of post-nasal voicing followed by assimilation to the second element: another process which perhaps began in Late antiquity, c.f. Horrocks, Geoffrey C. (2010). Greek: A history of the language and its speakers (2nd ed.). Oxford: Blackwell: 111, 171