The tenor voice is the highest male voice, except for the falsetto or otherwise produced register of the male alto and male soprano. In the Middle Ages the word had a different meaning. The tenor part of a vocal composition was the thematic basis, borrowed often from plainchant. The tenor voice came to assume the principal rôles in opera, largely replacing the castrato by the later 18th century. Various forms of tenor voice are demanded, particularly in opera, where the strong Heldentenor, (heroic tenor), met the requirements of Wagner, while other composers made use of lighter-voiced lyric tenors. The word tenor is also used adjectivally to describe instruments with a pitch lying between bass and alto, as, for example, the tenor trombone or, in earlier times, the tenor violin. The tenor clef, a C clef placed on the second line from the top of the five-line stave, is used for the upper registers of the cello and bassoon and for the tenor trombone.