When classifying in history, I understand that I must use --004 to designate ethnic or national groups. But what should I do for American history at 973? The instructions say ".01-.09 Standard subdivisions as modified under 930-990." Do I always use --004 or, in this case, --04?

The crux of the issue you outlined in your question is this: historical periods of the United States have no zeros (that is, they are in 973.1-973.9), unlike the history periods of all other countries, which are built on a single zero. Thus you use 973.01-973.09 for standard subdivisions of United States history as a whole, and 973.04 for ethnic and national groups in the United States.

Last revised: 30 December 2003