Unpredictable and originally defined by its fusion of styles with energy, rebellion, and catchy hooks, Rock & Roll is now known for its song-based music with more emphasis on its craftsmanship and live performance. The lyrics are often times romantic based, but do run the gauntlet of other themes including political and social influences. Rock usually has a 4/4 time signature, prominently featuring the electric guitar and vocals, supported by the electric bass, and percussion – drums and cymbals. Other instruments often complement the mix, including keyboards, synthesizers, and the acoustic guitar. Rock has evolved into many sub-genres, hybrids, and counter-cultures. Aerosmith, CCR, Rush, Kiss, Bruce Springsteen, Pantera, Soundgarden, and Nickelback are some of the Rock examples of this very diverse genre.