The guitar is often used to provide rhythmic and harmonic accompaniment to a voice or other instrument, or is used as an integral part of an ensemble. However, solo parts for the guitar are commonly found in a number of different musical styles. These can take the form of a section in which the guitar is heard more prominently than other instruments, or in which the guitar may be played entirely unaccompanied.
Unaccompanied guitar music is found in folk and classical music dating as far back as the instrument has existed, and the use of a guitar as a solo voice within an ensemble dates back at least to the Baroque concerto. The guitar was also important in jazz as well as being popular among blues musicians.
Guitar solo in rock music
Today the term guitar solo is commonly taken to refer specifically to the idiom of rock music. Although solo passages for guitar are found in many musical genres, the 'guitar solo' has an almost iconic place in rock, and particularly in the subgenres called hard rock or heavy metal.
Although in principle any kind of guitar can be used in a rock guitar solo, and counter-examples abound, the characteristic solo sound is created using an electric guitar with the timbral effect known as distortion, usually this effect is obtained by driving an amplifier with a higher signal than usual, but an effects pedal can be used to get similar effects. Rock bands sometimes have two guitarists, designated 'lead' and 'rhythm', the 'lead' player taking the solos while the 'rhythm' player accompanies. This arrangement is by no means universal, however.
Most examples of rock music are based around songs in very traditional forms. The main formal features are therefore verses, choruses, bridges and so on, all of which feature the voice as the most prominent instrument. Generally speaking, the guitar solo constitutes the only significant instrumental (that is, non-vocal) section of a mainstream rock song, although such songs often also have tutti sections in which the whole band plays one or more motifs known as riffs.
This use of an instrumental interlude to a song is of course very old, but probably the source in this case is performances by blues musicians, who were influential in the development of rhythm and blues and rock and roll, and hence more modern forms of rock music.
In most cases, the rock guitar solo is a short instrumental section of the song. In the classic verse-chorus form it quite often falls between the second chorus and third verse, although of course there are many variations. There are cases of extended guitar solos at the end of songs. In the progressive rock idiom, however, extended instrumental passages or even whole instrumental pieces became commonplace. In live performances, an extended guitar solo may be a frequent feature even if in recordings solos are usually kept short.
The accompaniment to the solo can vary. Probably the most common style is merely a continuation of verse and/or chorus instrumentation; in short, the solo is given the same backing as if it were a vocal passage. Other solos, often those used in songs with less conventional structures, feature an entirely new instrumental section; take for example 'Stairway to Heaven', which features the famous solo over elsewhere unused music. In that vein, some solos follow what could be described as the 'drop everything and solo' technique; the backing band stops playing entirely, giving a starker focus on the solo itself (the band often re-enters later in the solo), an examples of this is the solo from 'Heartbreaker'.
The use of the guitar solo in heavy metal music was especially notable during the 1980s, where a solo was a common feature for a guitar-based band and a lead guitarist who was highly-regarded might be as well-known as the singer. Later, guitarists who had developed considerable technical facility began to release albums which consisted only of guitar solos. This whole musical style, however, went out of fashion towards the end of the decade, and since then the guitar solo in pop and rock music declined in popularity; when present at all, solos tended to be more subdued and understated.
Musical content of rock guitar solos
Many famous guitarists are known primarily for their solos; as with any instrumentalist, styles vary considerably and so what follows can only be a generalisation. Most rock guitarists compose their solos, perhaps based initially on improvisations, and are able to reproduce them exactly when a song is performed live. This is important in the case of famous solos which come to be seen as much a part of the song as the sung parts; this contrasts strongly with jazz and blues guitarists, who typically do not compose their solos.
Since blues is the key source for rock music, the harmonic context of much traditional rock soloing is a single key-centre, without chromatic alterations. This, too, contrasts with jazz soloing in which 'running the changes', creating a melodic line that follows the underlying harmony, is a key part of the style. Because of this, the pitch-content of rock solos is generally based on scale choice rather than arpeggios of the underlying chords.
The minor pentatonic scale is a basic element of blues guitar solos. The idea is to contrast the flattened third and seventh scale degrees of the scale with the prevailing major tonality of the music underlying it. These so-called blue notes are often accentuated by 'bending' the pitch upwards towards the more consonant major third and root notes that lie above them. This gesture is characteristic of blues and jazz as well as rock.
Adding the second and (natural) sixth scale degrees to the minor pentatonic produces the so-called Dorian mode. This is also widely-used in rock guitar solos. Other minor modes of the major scale are also characteristic of the rock style, as is the harmonic minor scale, which is particularly associated with heavy metal.