Pop music is a genre of popular music that originated in its modern form in the United States and United Kingdom during the mid-1950s. The terms "popular music" and "pop music" are often used interchangeably, although the former describes all music that is popular and includes many different styles. "Pop" and "rock" were roughly synonymous terms until the late 1960s, when they became increasingly differentiated from each other. All though the genre is distinguished from chart music. Pop music is eclectic, and often borrows elements from other styles such as urban, dance, rock, Latin, and country; nonetheless, there are core elements that define pop music. Identifying factors include generally short to medium-length songs written in a basic format (often the verse-chorus structure), as well as common use of repeated choruses, melodic tunes, and hooks.
According to Frith, characteristics of pop music include an aim of appealing to a general audience, rather than to a particular sub-culture or ideology, and an emphasis on craftsmanship rather than formal "artistic" qualities. Music scholar Timothy Warner said it typically has an emphasis on recording, production, and technology, rather than live performance; a tendency to reflect existing trends rather than progressive developments; and aims to encourage dancing or uses dance-oriented rhythms. The main medium of pop music is the song, often between two and a half and three and a half minutes in length, generally marked by a consistent and noticeable rhythmic element, a mainstream style and a simple traditional structure. Common variants include the verse-chorus form and the thirty-two-bar form, with a focus on melodies and catchy hooks, and a chorus that contrasts melodically, rhythmically and harmonically with the verse. The beat and the melodies tend to be simple, with limited harmonic accompaniment.The lyrics of modern pop songs typically focus on simple themes – often love and romantic relationships – although there are notable exceptions. Harmony and chord progressions in pop music are often "that of classical European tonality, only more simple-minded." Clichés include the barbershop quartet-style harmony and blues scale-influenced harmony. There was a lessening of the influence of traditional views of the circle of fifths between the mid-1950s and the late 1970s, including less predominance for the dominant function.
The word "progressive" was frequently used, and it was thought that every song and single was to be a "progression" from the last. Music critic Simon Reynolds writes that beginning with 1967, a divide would exist between "progressive" pop and "mass/chart" pop, a separation which was "also, broadly, one between boys and girls, middle-class and working-class." Before the progressive pop of the late 1960s, performers were typically unable to decide on the artistic content of their music. Assisted by the mid-1960s economic boom, record labels began investing in artists, giving them freedom to experiment, and offering them limited control over their content and marketing. This situation fell in disuse after the late 1970s and would not reemerge until the rise of Internet stars. Indie pop, which developed in the late 1970s, marked another departure from the glamour of contemporary pop music, with guitar bands formed on the then-novel premise that one could record and release their own music without having to procure a record contract from a major label. In 2014, pop music worldwide was permeated by electronic dance music. A Scientific Reports study that examined over 464,000 recordings of popular music recorded between 1955 and 2010 found less variety in pitch progressions, growing average loudness levels, less diverse instrumentation and recording techniques, and less timbral variety, which declined after reaching a peak in the 1960s. Scientific American's John Matson reported that this "seems to support the popular anecdotal observation that pop music of yore was "better", or at least more varied, than today’s top-40 stuff."