As libraries only have a limited budget, and limited storage and display space, they cannot provide a comprehensive collection of resources for their members and communities. Even if they could, it would be massively wasteful as many resources are in low demand nationwide.
The solution libraries have adopted is to share resources through inter-library loan (ILL); this is sometimes known as interlending. Put simply, libraries borrow resources from each other to satisfy the requests of their members. It is not just books that are borrowed this way: talking books, vocal and orchestral performance sets, journal articles and other types of resources may be shared.
Public libraries have a particular strength when it comes to fiction, as through the Joint Fiction Reserve schemes, they co-operate in building a near-comprehensive collection of out-of-print fiction materials. These schemes can only work through the mechanism of ILL. It is not just public libraries that participate in ILL, though. Academic libraries, hospital libraries, and even some commercial companies' libraries share resources through ILL. National libraries are particularly prolific inter-library lenders, although they tend to specialize in academic publications.