Libraries are important in the program "linking" or "binding" process, which resolves references known as "links" or "symbols" to library modules. The linking process is usually automatically done by a "linker" or "binder" program that searches a set of libraries and other modules in a given order. Usually it is not considered an error if a link target can be found multiple times in a given set of libraries. Linking may be done when an executable file is created, or whenever the program is used at runtime.