Every project management trainee is (or should be) taught the triple constraints of time, scope, and cost. These constraints represent trade-offs; you can't change one without affecting the other two. For example, increasing scope has a corresponding effect on both project time and cost.
In this classic diagram, changing one leg of the triangle affects the other two:
The mysticMundane blog offers an unusual presentation of this fundamental set of project management relationships:
It may be obvious, but as Voltaire (and Will Rogers) have said, "Common sense isn't common"
If you're managing a project, it's imperative to understand the relationship between all sides of the triple constraint triangle. The seemingly obvious triple constraint laws are ignored every day, much to the chagrin of those responsible for the failed projects that result.