to the WBS Dictionary. A practical example of the WBS coding scheme is 1.0 Aircraft System 1.1 Air Vehicle 1.1.1 Airframe 1.1.1.1 Airframe Integration, Assembly, Test, and Checkout 1.1.1.2 Fuselage 1.1.1.3 Wing 1.1.1.4 Empennage 1.1.1.5 Nacelle 1.1.1.6 Other Airframe Components 1..n (Specify) 1.1.2 Propulsion 1.1.3 Vehicle Subsystems 1.1.4 Avionics 1.2 System Engineering 1.3 Program Management 1.4 System Test and Evaluation 1.5 Training 1.6 Data 1.7 Peculiar Support Equipment 1.8 Common Support Equipment 1.9 Operational/Site Activation 1.10 Industrial Facilities 1.11 Initial Spares and Repair Parts Terminal element The lowest element in a tree structure, a terminal element, is one that is not further subdivided. In a Work Breakdown Structure such elements (activity or deliverable), also known as work packages, are the items that are estimated in terms of resource requirements, budget and duration; linked by dependencies; and schedule. At the juncture of the WBS element and organization unit, control accounts and work packages are established, and performance is planned, measured, recorded, and controlled. A WBS can be expressed down to any level of interest. Three levels are the minimum recommended, with additional levels for and only for items of high cost or high risk, and two levels of detail at cases such as systems engineering or program management, with the standard showing examples of WBS with varying depth such as software development at points going to 5 levels or fire-control system to 7 levels. Consistent to norms The higher WBS structure should be consistent with whatever norms or template mandates exist within the organization or domain. For example, shipbuilding for the U.S. Navy must respect that the nautical terms and their hierarchy structure put into MIL-STD are embedded in Naval Architecture and that matching Navy offices and procedures have been built to match this naval architecture structure, so any significant change of WBS element numbering or naming in the hierarchy would be unacceptable. Example The adjacent figure shows a work breakdown structure construction technique that demonstrates the 100% rule and the "progressive elaboration" technique. At WBS Level 1 it shows 100 units of work as the total scope of a project to design and build a custom bicycle. At WBS Level 2, the 100 units are divided into seven elements. The number of units allocated to each element of work can be based on effort or cost; it is not an estimate of task duration. The three largest elements of WBS