How can a song belong to both folk music and popular music traditions?

A song can belong to both folk and popular music traditions because these categories are not discrete, mutually exclusive bodies of music, but rather two points on a musical continuum.1

The overlap occurs through several processes:

• Cross-fertilization: Societies often maintain both traditions, leading to the partial duplication of repertories and styles. A song may be considered folk due to its traditional roots or communal acceptance, yet also function as popular music when performed by professionals and disseminated via mass media for a nonparticipating urban audience.1
• Communal Re-creation: Folk songs are often subject to change as they pass through oral tradition. As they are performed, they may be adapted to sound more like contemporary popular styles.2
• Hybrid Genres: Styles such as folk rock emerged when artists fused traditional material with the technology and instrumentation of commercial pop.3
• Institutional Adoption: Throughout history, professional musicians have incorporated folk songs into their work, while popular music has frequently drawn upon folk themes, instruments, and melodies to evoke specific cultural identities.21