Constructing an overlay program involves manually dividing a program into self-contained [[object code]] blocks called '''overlays''' or '''links''', generally laid out in a [[tree structure]].{{efn|In [[OS/360 and successors]], there may be multiple ''regions''{{efn|This has nothing to do with the term ''region'' in [[OS/360 and successors#MVT|MVT]] storage management.}} each containing a complete overlay tree.}} ''Sibling'' segments, those at the same depth level, share the same memory, called ''overlay region''{{efn|The nomenclature varies depending on the system, e.g., in [[OS/360]] ''region'' refers to an entire overlay tree.}} or ''destination region''. An overlay manager, either part of the [[operating system]] or part of the overlay program, loads the required overlay from [[Auxiliary memory|external memory]] into its destination region when it is needed; this may be automatic or via explicit code. Often [[Linker (computing)|linker]]s provide support for overlays.<ref name="GNULink_2008"/>
