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What a WBS is: deliverable-oriented, decomposing scope to exactly 100% — no more, no less
A Work Breakdown Structure (WBS) is 'a deliverable-oriented hierarchical decomposition of the work to be executed by the project team' — all the work needed to accomplish the objectives and produce the required deliverables, broken down level by level, with each lower level defined in more detail.
It is the core artifact of scope management: only after the WBS spells out 'what things must be produced' can you talk about scheduling, estimating costs, assigning work, and managing risk. The WBS originated in US Department of Defense project practice (later standardized as MIL-STD-881) and was generalized by PMI's *Practice Standard for Work Breakdown Structures*.
⚠️The 100% rule is the most important principle of the WBS: the WBS must capture all the work defined by the project scope — every deliverable (external, internal, interim), including project management work itself — and must not include any work outside the scope. And the rule holds at every level: the children of any parent node must add up to exactly 100% of the parent, no more, no less — 'under 100% means work nobody owns (guarding against gaps); over 100% means doing work that was never required (guarding against gold-plating / scope creep).'
