rhythm guitars. Riffs In some cases, the chord progression is implied with a simplified sequence of two or three notes, sometimes called a "riff". That sequence is repeated throughout the composition. In heavy metal music, this is typically expanded to more complex sequences comprising a combination of chords, single notes and palm muting. The rhythm guitar part in compositions performed by more technically oriented bands often include riffs employing complex lead guitar techniques. In some genres, especially metal, the audio signal from the rhythm guitar's output is often subsequently heavily distorted by overdriving the guitar's amplifier to create a thicker, "crunchier" sound for the palm-muted rhythms. Interaction with other guitarists In bands with two or more guitarists, the guitarists may exchange or even duplicate roles for various songs or several sections within a song. In those with a single guitarist, the guitarist may play lead and rhythm at numerous times or simultaneously, by overlaying the rhythm sequence with a lead line. Crossover with keyboards The availability of electronic effects units such as delay pedals and reverb units enables electric guitarists to play arpeggios and take over some of the role of a synthesizer player in performing sustained "pads". Those serve as sonic backgrounds in modern pop. Creating a pad sound differs from usual rhythm guitar roles in that it is not rhythmic. Some bands have a synthesizer performer play pads. In bands without a synth player, a guitarist can take over this role. Replacing lead guitar Some rhythm techniques cross over into lead guitar playing. In guitar-bass-and-drums power trios guitarists must double up between rhythm and lead. For instance Jimi Hendrix combined full chords with solo licks, double stops and arpeggios. In the 2010s, "looping pedals" are used to record a chord sequence or riff over which musicians can then play the lead line, simulating the sound achieved by having two guitarists. Equipment Rhythm guitarists usually aim to generate a stronger rhythmic and chordal sound, in contrast to the lead guitarists' goal of producing a sustained, high-pitched melody line that listeners can hear over the top of the band. As a result, rhythm and lead players may use different guitars and amplifiers. Rhythm guitarists may employ an electric acoustic guitar or a humbucker-equipped electric guitar for a richer and fatter output. Also, rhythm guitarists may use strings of a larger gauge than those used by lead guitarists. However, while these