The Hierarchy of Constraints
We’ve determined that one of the most powerful secrets of the triple constraints comes from ranking them in the order of their priority and impact on the project—that is, deciding which pig is more equal than the other two.
The three terms we use to describe the order of the triple constraints in a hierarchy are:
Driver. The driver is the constraint that cannot fail without dragging the project down to failure along with it. The flexibility of the driver is not necessarily zero, but it is less than the flexibility of the other two constraints. The consequence of failure is serious, visible, and not excusable ...