Rock music today is an unclear defined genre of popular music that developed from the classic 1960s Rock and Roll music. It has its roots in rockabilly, which in turn evolved from rhythm and blues, country music and other musical influences. Rock music is widely popular and available on music CDs and through MP3 downloads and continues to incorporate different musical elements including folk music, jazz music and even classical music. The centre piece of many Rock music sounds are based on guitar and the classic Rock rhythm based on the human heart rhythm. To the day there are many Rock bands usually featuring a singer or singer duo, guitarists and keyboard players. Other instruments and music artists make up the picture.