A music genre is a conventional category that identifies some pieces of music as belonging to a shared tradition or set of conventions. It is to be distinguished from musical form and musical style, although in practice these terms are sometimes used interchangeably. Recently, academics have argued that categorizing music by genre is inaccurate and outdated.
Music can be divided into different genres in many different ways. The artistic nature of music means that these classifications are often subjective and controversial, and some genres may overlap. There are even varying academic definitions of the term genre itself. In his book Form in Tonal Music, Douglass M. Green distinguishes between genre and form. He lists madrigal, motet, canzona, ricercar, and dance as examples of genres from the Renaissance period. To further clarify the meaning of genre, Green writes, "Beethoven's Op. 61 and Mendelssohn's Op. 64 are identical in genre – both are violin concertos – but different in form. However, Mozart's Rondo for Piano, K. 511, and the Agnus Dei from his Mass, K. 317 are quite different in genre but happen to be similar in form." Some, like Peter van der Merwe, treat the terms genre and style as the same, saying that genre should be defined as pieces of music that share a certain style or "basic musical language." Others, such as Allan F. Moore, state that genre and style are two separate terms, and that secondary characteristics such as subject matter can also differentiate between genres. A music genre or subgenre may also be defined by the musical techniques, the style, the cultural context, and the content and spirit of the themes. Geographical origin is sometimes used to identify a music genre, though a single geographical category will often include a wide variety of subgenres. Timothy Laurie argues that since the early 1980s, "genre has graduated from being a subset of popular music studies to being an almost ubiquitous framework for constituting and evaluating musical research objects".
Among the criteria often used to classify musical genres are the trichotomy of art, popular, and traditional musics.
Alternatively, music can be divided on three variables: arousal, valence, and depth. Arousal reflects the energy level of the music; valence reflects the scale from sad to happy emotions, and depth reflects the level of emotional depth in the music. These three variables help explain why many people like similar songs from different traditionally segregated genres.
Musicologists have sometimes classified music according to a trichotomic distinction such as Philip Tagg's "axiomatic triangle consisting of 'folk', 'art' and 'popular' musics". He explains that each of these three is distinguishable from the others according to certain criteria.
The term art music refers primarily to classical traditions, including both contemporary and historical classical music forms. Art music exists in many parts of the world. It emphasizes formal styles that invite technical and detailed deconstruction and criticism, and demand focused attention from the listener. In Western practice, art music is considered primarily a written musical tradition, preserved in some form of music notation rather than being transmitted orally, by rote, or in recordings, as popular and traditional music usually are. Historically, most western art music has been written down using the standard forms of music notation that evolved in Europe, beginning well before the Renaissance and reaching its maturity in the Romantic period. The identity of a "work" or "piece" of art music is usually defined by the notated version rather than by a particular performance, and is primarily associated with the composer rather than the performer (though composers may leave performers with some opportunity for interpretation or improvisation). This is so particularly in the case of western classical music. Art music may include certain forms of jazz, though some feel that jazz is primarily a form of popular music.
Sacred Christian music forms an important part of the classical music tradition and repertoire, but can also be considered to have an identity of its own.
The term popular music refers to any musical style accessible to the general public and disseminated by the mass media. Musicologist and popular music specialist Philip Tagg defined the notion in the light of sociocultural and economical aspects:
Popular music, unlike art music, is (1) conceived for mass distribution to large and often socioculturally heterogeneous groups of listeners, (2) stored and distributed in non-written form, (3) only possible in an industrial monetary economy where it becomes a commodity and (4) in capitalist societies, subject to the laws of 'free' enterprise ... it should ideally sell as much as possible.
Popular music is found on most commercial and public service radio stations, in most commercial music retailers and department stores, and in movie and television soundtracks. It is noted on the Billboard charts and, in addition to singer-songwriters and composers, it involves music producers more than other genres do.
The distinction between classical and popular music has sometimes been blurred in marginal areas such as minimalist music and light classics. Background music for films/movies often draws on both traditions. In this respect, music is like fiction, which likewise draws a distinction between literary fiction and popular fiction that is not always precise.
Traditional music is a modern name for what has been called "folk music", excluding the expansion of the term folk music to include much non-traditional material. Sometimes "folk" is designated for Western music and non-Western music is considered "world music". The two are both unified as traditional music due to:
Musicologist and popular music specialist Richard Middleton has discussed the blurred nature of these distinctions:
Neat divisions between 'folk' and 'popular', and 'popular' and 'art', are impossible to find ... arbitrary criteria [are used] to define the complement of 'popular'. 'Art' music, for example, is generally regarded as by nature complex, difficult, demanding; 'popular' music then has to be defined as 'simple', 'accessible', 'facile'. But many pieces commonly thought of as 'art' (Handel's 'Hallelujah Chorus', many Schubert songs, many Verdi arias) have qualities of simplicity; conversely, it is by no means obvious that the Sex Pistols' records were 'accessible', Frank Zappa's work 'simple', or Billie Holiday's 'facile'.
New genres can arise by the development of new forms and styles of music and also simply by creating a new categorization. Although it is conceivable to create a musical style with no relation to existing genres, new styles usually appear under the influence of preexisting genres. The genealogy of musical genres expresses, often in the form of a written chart, the way in which new genres have developed under the influence of older ones. If two or more existing genres influence the emergence of a new one, a fusion between them can be said to have taken place. The proliferation of popular music in the 20th century has led to over 1,200 definable subgenres of music.