Binary form is usually characterized as having the form "AB", though since both sections repeat, a more accurate description would be "AABB". Others, however, prefer to use the label "AA′". This second designation points to the fact that there is no great change in character between the two sections. The rhythms and melodic material used will generally be closely related in each section, and if the piece is written for a musical ensemble, the instrumentation will generally be the same. This is in contrast to the use of verse-chorus form in popular music—the contrast between the two sections is primarily one of the keys used.