In common usage, alto is used to describe the voice type that typically sings this part, though this is not strictly correct: alto , like the other three standard modern choral voice classifications (soprano, tenor and bass) was originally intended to describe a part within a homophonic or polyphonic texture, rather than an individual voice type Stark (2003), Bel Canto: A History of Vocal Pedagogy ; neither are the terms alto and contralto interchangeable or synonymous, though they are often treated as such. Although some women who sing alto in a choir are contraltos, many would be more accurately called mezzo-sopranos (a voice of somewhat higher range and different timbre), and many men countertenors (this latter term is a source of considerable controversy, some authorities preferring the usage of the term "male alto" for those countertenors who use a predominantly falsetto voice production). The contralto voice is a matter of vocal timbre and vocal tessitura as well as range, and a classically-trained solo contralto would usually have a range greater than that of a normal choral alto part in both the upper and lower ranges. However, the vocal tessitura of a classically trained contralto would still make these singers more comfortable singing in the lower part of the voice. A choral non-solo contralto may also have a low range down to D3 (thus perhaps finding it easier to sing the choral tenor part), but some would have difficulty singing above E5. In a choral context mezzo-sopranos and contraltos might sing the alto part, together with countertenors, thus having three vocal timbres (and two means of vocal production) singing the same notes. Smith (2005), Choral Pedagogy Alto is rarely used to describe a solo voice, though there is a plethora of terms in common usage in various languages and in different cultures for solo singers in this range. Examples include contralto, countertenor, haute-contre , and tenor altino among others.
Source: Wikipedia > Alto
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