How does a cantus firmus serve as the basis for a fugue?

In the context of music, a cantus firmus serves as a foundational, preexistent melody—such as a plainchant or a chorale—upon which a fugue is constructed.12

When used as the basis for a fugue, the cantus firmus provides a fixed melodic framework that is integrated into the contrapuntal texture of the composition. Composers often incorporate the cantus firmus by presenting it in widely spaced phrases, allowing the fugal voices to develop around it. This technique creates a structural layer that supports the complex interplay of the fugue's subjects and countersubjects.12

Historically, this approach has been used to lend symbolic or traditional weight to a work, as seen in the compositions of J.S. Bach, where chorale melodies are superposed within a fugal structure to build toward a climax. By combining the rigorous imitative development characteristic of a fugue with the stability of a preexistent melody, the cantus firmus acts as a unifying element that anchors the polyphonic complexity of the piece.12