What is a Triple Constraint (Iron Triangle)?
The triple constraint — scope, time, cost, with quality living in the middle — says the three sides are linked: change one and at least one other must move. More scope needs more time or money; less time costs more money or scope. Modern PMI expands the list (risk, resources, quality as peers), but the triangle remains the fastest way to reason about trade-offs.
Its exam use is discipline: when a stakeholder demands a change to one side "for free," your job is to show which other side just moved.
Worked example
A client wants the store opening moved up three weeks (time ↓). The contractor offers the triangle: add night shifts (+$90K, cost ↑) or defer the mezzanine fit-out to after opening (scope ↓). What the triangle forbids is the fourth option everyone wants: same scope, same cost, three weeks faster.