Art music can be defined as serious music with a written musical tradition. The term art music is used often and has a clear meaning. Musicologist Philip Tagg calls it one part of an clear and unquestioned "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 main tradition in the Western world is usually called classical music. In this regard, it is frequently used as a contrasting term to popular music and traditional or folk music.