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 Recently, academics have argued that categorizing music by genre is inaccurate and outdated.Music can be divided into different genres in many different ways.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.