The Triple Constraint Diagram, also referred to as the Project Management Triangle or Iron Triangle, is essentially an overview of the constraints each project faces in completing on time, on budget and on scope. Essentially, if you alter one leg of the triangle it invariably impacts the other legs of the triangle. So, if you have a slip in schedule either the scope, cost or both need to change in order to compensate for those and achieve the same balance of project quality. This is useful as it helps demonstrate to the Project Team the need to balance the three through the execution of the project and if either cost, scope or schedule are impacted it doesn't happen independent of the others and those should be closely reviewed to identify what those impacts are, if they are acceptable and how they can be managed.
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Useful Links
Project Management Triangle - Wikipedia