A music genre is a categorical and typological construct that identifies musical sounds as belonging to a particular category and type of music that can be distinguished from other types of music. Genre is a common concept that has great commercial and aesthetic importance, but it also continues to frustrate fans, artists, composers, and critics who don't want to be pigeonholed. There is always disagreement about the definition of a genre, and it is impossible to list all genre categories in existence. For further examination of these general issues, see Negus 1999 and Holt 2007 (reference list below).