In comparison, a reference book or reference-only book in a library is one that may only be used in the library and may not be borrowed from the library. Many such books are reference works (in the first sense), which are, usually, used briefly or photocopied from, and, therefore, do not need to be borrowed. Keeping them in the library assures that they will always be available for use on demand. Some reference-only books are too valuable to permit borrowers to take them out. Reference-only items may be shelved in a reference collection located separately from circulating items. Some libraries consist entirely, or to a large extent, of books which may not be borrowed