I learned to use the library's catalogues* at an annoyingly early age. Certain pages were stamped with a large, red C. S. I soon worked out that these were the items which did not appear on the shelves. I should have worked out that it meant Closed Shelves but I imagined it meant Cold Storage. The books and records I delighted in sending the librarians to retrieve were always rather cold, so I imagined they did come from a cellar!
I know that some libraries used the Closed Shelves system to protect the eyes of the public from risqué material. Maybe I should have explored beyond the unpopular Russian operas it was my delight to have called back from Hades!
*These were solid wooden block-type binders, bound in maroon leather or leatherette, much wider than they were high. Entries were monotyped on India-paper with red and black ribbon. The assembly could be unscrewed to interpose new leaves. I mention this, since many may never have encountered them! They may exist in some libraries still but I have never had to use one in decades.