Studying large plainchant corpora using chant21

We present chant21, a Python package to support the plainchant formats gabc and Volpiano in music21, and two large corpora of plainchant. The CantusCorpus contains over 60,000 medieval melodies collected from the Cantus database, encoded in the Volpiano typeface. The GregoBaseCorpus contains over 9,000 transcriptions from more recent chant books in the gabc format. Chant21 converts both formats to music21, while retaining the textual structure of the chant: its division in sections, words, syllables and neumes. We present two case studies. First, we report evidence for the melodic arch hypothesis from the GregoBaseCorpus. Second, we analyze connections between differentiæ and antiphon openings in the CantusCorpus, and show that the systematicity of the connection can be quantified using an entropy-based measure.