Music can be divided into different genres in several ways. The artistic nature of music means that these classifications are often arbitrary and controversial, and some genres may overlap. There are several academic approaches to genres. In his book "Form in Tonal Music", Douglass M. Green lists madrigal, motet, canzona, ricercar, and 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. 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".