Like a plainchant from liturgical music, a cantus firmus is a pre-existing melody that forms the basis for a polyphonic composition (one with several independent voices or parts), and it was used as a compositional tool. During the 11th and 12th century, a simple second melody, called a duplum, started to be added to an existing plainchant melody (the vox principalis, or main voice). The 13th – century polyphonic motet featured the plainchant cantus firmus in the tenor. “Tenor” derives from Latin tenere, “to hold”— i.e., the voice part that holds the plainchant. During the Renaissance, masses and motets commonly featured a cantus firmus in the tenor, which was by then no longer the lowest voice. At times, though, the cantus firmus appeared ornamented or paraphrased in the top voice. The plainchant had symbolic as well as purely musical connotations.
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