[PW] Origin of the term "stacks" (in the library)

[ALAN GREEN]

One of our professors would like to know, why do we call them "stacks" (i.e., the book stacks)?  Did libraries formerly stack books flat in piles on the shelves (or the floor, or on tables), before moving to the current vertical shelving system?

The "Stack" entry in OED provided no help for this question.  Please copy replies to my address, as I am not a list subscriber.

Thanks!

Alan Green
Music Librarian, Ohio State University
green.200 at osu.edu