In the Q&A session for one of our recent webinars (DDC 23 Update), the following question was posed: “Are you able to give some insight into the reasoning behind relocating comprehensive works on rhythm & blues to 781.644 (the number for Soul music) from the number for blues (781.643)?”
Here is how the two entries read in DDC 23:
Class here traditional rhythm and blues; specific blues styles
Comprehensive works on rhythm and blues relocated to 781.644
Class here comprehensive works on rhythm and blues [formerly 781.643], R&B; specific soul styles
For traditional rhythm and blues, see 781.643
That is, traditional rhythm and blues are part of 781.643 Blues, while contemporary rhythm and blues are part of 781.644 Soul. Comprehensive works on rhythm and blues—works on both traditional and contemporary rhythm and blues—have been relocated from 781.643 Blues to 781.644 Soul.
In our review of popular music styles in the period before the publication of DDC 23, we investigated changing the structure at 781.643 Blues and 781.644 Soul to parallel the overall structure of allmusic.com, which distinguishes between blues and R&B and which treats soul music as a subcategory under R&B. The arrangement in allmusic.com differs subtly from the meaning afforded these terms in Library of Congress Subject Headings, with its entries for Blues (Music), Rhythm and blues music (UF R&B), and Soul music. Both Blues (Music) and Soul music are RTs of Rhythm and blues music (and vice-versa). (For the record, the Library of Congress Classification combines Blues and Rhythm and blues into a single class.)
WorldCat searches showed that Blues (Music) had been assigned quite infrequently in comparison to Rhythm and blues music and Soul music. At the same time, works assigned the LCSH Rhythm and blues music were much more likely to be classed in 781.643 Blues than in 781.644 Soul. It appears from this classification practice that the class-here note  for comprehensive works on rhythm and blues that had been in 781.643 was pulling literature into that class that should rightfully have gone into 781.644, given the USE/UF relationship between Rhythm and blues music and R&B.
Our solution has been (1) to distinguish between traditional rhythm and blues, aligned with 781.643 Blues, and contemporary rhythm and blues (that is, R&B), aligned with 781.644 Soul and (2) to relocate comprehensive works on rhythm and blues to 781.644. The first part of our solution accords with the distinction between blues and R&B made on allmusic.com; the second part of our solution aligns the DDC’s comprehensive works on rhythm and blues with the LCSH Rhythm and blues music and corrects past misclassifications based on the assignment of that LCSH.