The next book I want to classify is "Editing Early Music" by John Caldwell (ISBN 0-19-816544-7). The publisher, Oxford University Press, claims that it should be filed at 781.2 Elements of Music but this doesn't seem right to me. I'm not sure which edition of the Dewey Classification system they are using, perhaps this number made more sense before the great revision. Looking at the revised 780 schedule I found 780.149 Editing Music. This seems to be what I'm looking for. There is also a notation under 780.148 Musical notation, abbreviations, symbols that says "Class transcription from one form of notation to another in 780.149". This again indicates that I'm on the right track (this book is all about transforming early music notation into a form that can be understood by a modern musician).

At this point I could easily say I'm done but the Dewey system does allow further aspects or facets of the work to be captured. In this case I think you could add 780.902 500 - 1449 to the base number to indicate that the book is relevant to early music. The way you do this is still a little unclear to me. I believe that you add a facet indicator (which can be either 0 or 1) to the base number and then append the number after the decimal place in the facet. This would yield either 780.1490902 or 780.1491902. I think this would be fine in a large library with a ton of books on the same subject. For my library (with maybe 50 books and a couple hundred scores) I think it makes more sense to use the shorter classification number and then sort all the books in that class by Cutter number.

Update: I now believe that 0 is used to indicate a standard subdivision and 1 indicates a facet for a built number. So, in the example above, 780.1491902 would be the correct class number. Please correct me if I'm wrong.