Music is an art form whose medium is sound and silence. Common elements of music are pitch (which governs melody and harmony), rhythm (and its associated concepts tempo, meter, and articulation), dynamics, and the sonic qualities of timbre and texture. A musician is an artist who writes, performs, or creates music. Musicians can be classified by their roles in creating or performing music. A singer is a person who uses his or her voice to produce music. Singers are often accompanied by musicians and instruments, while other people sing to have fun. Vocal skill is usually a combination of innate talent and professional training. Singers are also referred to as vocalists.