The category is "Best Play" which I believe would refer to the script.
For a few years in the early 60s there was a separate category for playwrights (Neil Simon won his first Tony in this category), but what did it mean? The issue was settled in about 1965 by the ATW with the decision that the Best Play was also the best written play and the Best Author category was discontinued.
For that matter there was also a separate category for Best Producer and what does that mean?
With a musical there was the decision that the writing components were indeed separate contributions and could be honored thusly. These weren't finalized until 1972.
1776 and Applause (1969 and 1970) were awarded Tonys not to just the producers but to all the authors as well. Hallelujah Baby was given Tonys to the producers and the book writer, but the score was in a separate competitive category.
The other categories cover the production values.
Does any of this make sense? It is all from memory so I hope the facts are right.
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