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runscott"2. a part added to a book or an extra part of a newspaper."
A "Sporting News" supplement is a good example of what a supplement is. But, Scott, to use your example - by your definition every full-page plate in a Burr-McIntosh could be thought of as a supplement since it was common for people to take it apart and display the pages; however, a true supplement would be either a section added to the already complete item, or a page inserted into the already complete item...I would even agree that we now commonly use the word "supplement" to describe the large photos that you could mail off for using ads in publications. On the other hand, I would not consider the inside front cover of a copy of "Baseball Magazine" to be a supplement, even though it might have a photo of a baseball player and also the appearance of a true "Baseball Magazine" supplement.
I don't think it's a gray area at all.