the WBS also helps map requirements from one level of system specification to another, for example, a cross-reference matrix mapping functional requirements to high level or low-level design documents. The WBS may be displayed horizontally in outline form or vertically as a tree structure (like an organization chart). The development of the WBS normally occurs at the start of a project and precedes detailed project and task planning. History The concept of work breakdown structure was developed with the Program Evaluation and Review Technique (PERT) by the United States Department of Defense (DoD). PERT was introduced by the U.S. Navy in 1957 to support the development of its Polaris missile program. While the term "work breakdown structure" was not used, this first implementation of PERT did organize the tasks into product-oriented categories. By June 1962, DoD, NASA, and the aerospace industry published a document for the PERT/COST system, which described the WBS approach. This guide was endorsed by the Secretary of Defense for adoption by all services. In 1968, the DoD issued "Work Breakdown Structures for Defense Materiel Items" (MIL-STD-881), a military standard requiring the use of work breakdown structures across the DoD. The document has been revised several times, most recently in 2018. The current version of this document can be found in "Work Breakdown Structures for Defense Materiel Items" (MIL-STD-881E). It includes WBS definitions for specific defense materiel commodity systems and addresses WBS elements that are common to all systems. Defense Materiel Item categories from MIL-STD-881E are: Aircraft Systems Electronic/Generic Systems Missile/Ordnance Systems Strategic Missile Systems Sea Systems Space Systems Ground Vehicle Systems Unmanned Maritime Systems Launch Vehicle Systems Information Systems/Defense Business Systems The common elements identified in MIL-STD-881E, Appendix K are: Integration, assembly, test, and checkout; Systems engineering; Program management; System test and evaluation; Data; Peculiar support equipment; Common support equipment; Operational/Site activation; Contractor Logistics Support; Industrial facilities; Initial spares and repair parts. The standard also includes additional common elements unique to Space Systems, Launch Vehicle Systems, and Strategic Missile Systems. In 1987, the Project Management Institute (PMI) documented expanding these techniques across non-defense organizations. The Project Management Body of Knowledge (PMBOK) Guide provides an overview of the WBS concept, while the "Practice Standard for Work Breakdown Structures" is comparable to the DoD standard but is intended for more general application. Design principles 100% rule An important design principle for work breakdown structures is called the 100%