may be practices, they are not necessarily the rule and are subject to the style of the song and the preference of the individual guitarist. While rhythm guitarists in metal bands use distortion effects, they tend to use less of the modulation effects such as flangers used by lead guitar players. Whereas the lead guitarist in a metal band is trying to make the solo tone more prominent, and thus uses a range of colorful effects, the rhythm guitarist is typically trying to provide a thick, solid supporting sound that blends in with the overall sound of the group. In alternative rock and post punk bands, however, where the band is trying to create an ambient soundscape rather than an aggressive Motörhead-style "Wall of Sound", the rhythm guitarist may use flanging and delay effects to create a shimmering background. Jazz Rhythm guitar has been especially important in the development of jazz. The guitar took over the role previously occupied by the banjo to provide rhythmic chordal accompaniment. Early jazz guitarists like Freddie Green tended to emphasize the percussive quality of the instrument. The ability to keep a steady rhythm while playing through complicated chord patterns made the guitar invaluable to many rhythm sections. Jazz guitarists are expected to have deep knowledge of harmony. Jazz harmony Jazz guitarists use their knowledge of harmony and jazz theory to create jazz chord "voicings", which emphasize the 3rd and 7th notes of the chord. Unlike pop and rock guitarists, who typically include the root of a chord (even, with many open chords and barre chords, doubling the root), jazz guitarists typically omit the root. Some more sophisticated chord voicings also include the 9th, 11th, and 13th notes of the chord. A typical jazz voicing for the chord G7 would be the individual notes B, E, F, and A. This voicing uses the 3rd (the note B), the 7th (the note F), along with the 6th (the note E) and the 9th (the note A). In some modern jazz styles, dominant 7th chords in a tune may contain altered 9ths (either flattened by a semitone, which is called a "flat 9th", or sharpened by a semitone, which is called a "sharp 9th"); 11ths (sharpened by a semitone, which is called a "sharp 11th"); 13ths (typically flattened by a semitone, which is called a "flat 13th"). Jazz guitarists need to learn about a range of different