The catalogue was made by Wolfgang Schmieder in 1950. He took all the music that is known by Johann Sebastian Bach (some of Bach's music has been lost) and gave each piece a number. Each piece of music by Bach therefore has a BWV number. For example, the "Mass in B minor" is BWV 232. Giving each piece a number helps musicians to know exactly which piece of music we are talking about. For example, there are several Preludes and Fugues in G major, but putting "Prelude and Fugue in G major BWV550" makes it clear which one it is.