||A good definition of rock, in fact, is that it's popular music that to a certain degree doesn't care if it's popular.
Red Hot Chili Peppers
in 2006, showing a quartet lineup for a rock band (from left to right: bassist, lead vocalist, drummer, and guitarist).
The sound of rock is traditionally centered on the
electric guitar, which emerged in its modern form in the 1950s with the popularization of rock and roll,
 and was influenced by the sounds of
electric blues guitarists.
 The sound of an electric guitar in rock music is typically supported by an electric
bass guitar, which pioneered in jazz music in the same era,
 and percussion produced from a
drum kit that combines drums and cymbals.
 This trio of instruments has often been complemented by the inclusion of other instruments, particularly keyboards such as the
Hammond organ and
 The basic rock instrumentation was adapted from the basic
blues band instrumentation (prominent
lead guitar, second chordal instrument, bass, and drums).
 A group of musicians performing rock music is termed a
rock band or
rock group and typically consists of between three–the
power trio–and five members. Classically, a rock band takes the form of a
quartet whose members cover one or more roles, including
drummer and often that of
keyboard player or other
Rock music is traditionally built on a foundation of simple unsyncopated rhythms in a 4/4
meter, with a repetitive snare drum
back beat on beats two and four.
 Melodies are often derived from older
musical modes, including the
Mixolydian, as well as
minor modes. Harmonies range from the common
triad to parallel
fifths and dissonant harmonic progressions.
 Rock songs, since the late 1950s
 and particularly from the mid-1960s onwards, often used the
verse-chorus structure derived from blues and folk music, but there has been considerable variation from this model.
 Critics have stressed the eclecticism and stylistic diversity of rock.
 Because of its complex history and tendency to borrow from other musical and cultural forms, it has been argued that "it is impossible to bind rock music to a rigidly delineated musical definition."
A simple 4/4 drum pattern common in rock music
Unlike many earlier styles of popular music, rock lyrics have dealt with a wide range of themes in addition to romantic love: including sex, rebellion against "
The Establishment", social concerns and life styles.
 These themes were inherited from a variety of sources, including the
Tin Pan Alley pop tradition, folk music and rhythm and blues.
 Music journalist
Robert Christgau characterizes rock lyrics as a "cool medium" with simple diction and repeated refrains, and asserts that rock's primary "function" "pertains to music, or, more generally,
 The predominance of white, male and often middle class musicians in rock music has often been noted
 and rock has been seen as an appropriation of black musical forms for a young, white and largely male audience.
 As a result, it has been seen as articulating the concerns of this group in both style and lyrics.
 Christgau, writing in 1972, said in spite of some exceptions, "rock and roll usually implies an identification of male sexuality and aggression".
Since the term rock began to be used in preference to rock and roll from the late-1960s, it has often been contrasted with pop music, with which it has shared many characteristics, but from which it is often distanced by an emphasis on musicianship, live performance and a focus on serious and progressive themes as part of an ideology of authenticity that is frequently combined with an awareness of the genre's history and development.
 According to
Simon Frith "rock was something more than pop, something more than rock and roll. Rock musicians combined an emphasis on skill and technique with the romantic concept of art as artistic expression, original and sincere".
 In the new millennium the term rock has sometimes been used as a
blanket term including forms such as
soul music, and even
hip hop, with which it has been influenced but often contrasted through much of its history.