In music, one voice per part or one vocalist per part (OVPP) is the practice of performing choral music with a single voice on each vocal line. In the specific context of Johann Sebastian Bach's works, it is also known as the Rifkin hypothesis, as set forth in a 1982 article by Joshua Rifkin and expanded in Andrew Parrott's book The Essential Bach Choir. Choral works featuring SATB vocal parts are consequently sung by only four singers when this approach is adopted.