Written notation varies with style and period of music. In the 2000s, notated music is produced as sheet music or, for individuals with computer scorewriter programs, as an image on a computer screen. In ancient times, music notation was put onto stone or clay tablets. To perform music from notation, a singer or instrumentalist requires an understanding of the rhythmic and pitch elements embodied in the symbols and the performance practice that is associated with a piece of music or a genre. In genres requiring musical improvisation, the performer often plays from music where only the chord changes and form of the song are written, requiring the performer to have a great understanding of the music's structure, harmony and the styles of a particular genre (e.g., jazz or country music).