jats:titleAbstract</jats:title> jats:pJ. S. Bach’s Well-Tempered Clavier II is well known for the complexity of its source situation, and its vast array of variant readings. The current article uses techniques of phylogenetic analysis, developed in the biological sciences, to deepen our understanding of the complex relationships between the primary sources. The computer algorithm NeighborNet is used to analyse data comprising the textual variants for the Prelude in A, bwv888 and the Prelude in C, bwv870. The resultant grouping of sources reflects the differences in revision practice between the two preludes. While Bach saw little need to revise the Prelude in A, the Prelude in C underwent a process of continued revision that can be discerned in the results of the phylogenetic analysis. The analyses also highlight the uncertain relationship of the manuscript DD70 with the other sources of the Prelude in C and the implications for 18th-century performance practice.</jats:p>