Johann Sebastian Bach knew the value of good compositions by other composers, as is evidenced by contemporary accounts and by looking through the remains of his music library. However, in certain instances when copying works which he prepared for his own use he could seldom refrain from making corrections.

In both of the following movements of the Missa Sancti Lambertie by Johann Christoph Pez Bach made only a few changes. With regard to his motive for copying out these rather simple movemen...