The historical development of medieval plainchant melodies is an intriguing musicological topic that invites computational approaches to study it at scale. Plainchant melodies can be represented as strings from a limited alphabet, hence making it technically possible to apply bioinformatic tools that are used to study the relationships of biological sequences. We show that using phylogenetic trees to study relationships of plainchant sources is not merely possible, but that it can indeed produce meaningful results. We develop a simple plainchant substitution model for Multiple Sequence Alignment, adapt a Bayesian phylogenetic tree building method, and demonstrate the promise of this approach by validating the resultant phylogenetic tree built from a set of Divine Office sources for the Christmas Vespers against musicological knowledge.

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