Believed to have been dictated by J.S. Bach to one of his students at the Leipzig Thomas-Schule, the Precepts and Principles for Playing the Thorough Bass or Accompanying in Four Parts is the most important teaching document of Bach's to have survived, providing insights into his teaching ofharmony and fugue, and perhaps on his compositional process as well. Heinrich Schenker made use of Precepts (which he called Bach's Generalbassbuchlein) in the formulation of his analytical system.Here is a translation, commentary, transcription of the music examples into modern notation, and complete facsimile of this treatise. The 48-leaved manuscript, in which the hand of Bach's pupil Carl August Thieme (1721-95) has been identified, consists of thorough bass rules for writing andplaying in four parts, studies in fugue writing, as well as figured basses and cadence patterns suitable for keyboard practice.