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.