What is the triple constraint in project management?

The triple constraint, also called the iron triangle, describes the interdependence of scope, schedule, and cost in every project. Changing one constraint inevitably affects one or both of the others. For example, increasing scope without extending the schedule requires more resources, increasing cost. Reducing the budget without reducing scope requires extending the timeline. Quality sits at the center because it is affected by the balance of all three. The project manager's job is to manage these competing constraints according to stakeholder priorities.