A music genre is a term that describes the process of dividing different kinds of music into categories. These categories vary by definition of the word "genre," and there appears to be several definitions. In his book Form in Tonal Music, Douglass M. Green lists the madrigal, the motet, the canzona, the ricercar, and the dance as examples of genres (from the Renaissance period). According to Green, "Beethoven's Op. 61 and Mendelssohn's Op. 64 are identical in genre - both are violin concertos - but different in form. 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 treat the terms genre and style as the same, and state that genre should be defined as pieces of music that share a certain style or "basic musical language." Peter van der Merwe 1989, p.3 Others state that genre and style are two separate terms, and that secondary characteristics such as subject matter can also differentiate between genres. Moore, Allen "Categorical Conventions in Music Discourse: Style and Genre" Music & Letters, Vol. 82, No. 3 (Aug., 2001), pp. 432-442 A music genre (or sub-genre) could be defined by the techniques, the styles, the context and the themes (content, spirit). Also, geographical origin is sometimes used to define the music genre, though a single geographical category will normally include a wide variety of sub-genres.