A drum machine is an electronic musical instrument designed to imitate the sound of drums and/or other percussion instruments. Drum machines are very useful instruments for a wide variety of musical genres, not just purely electronic music. They are also a common necessity when session drummers are not available or desired. Most modern drum machines are sequencers with a sample playback (rompler) or synthesizer component that specializes in the reproduction of drum timbres as well as the sound of other traditional percussion instruments. Though features vary from model to model, many modern drum machines can also produce unique sounds (though usually percussive in nature), and allow the user to compose unique drum beats.