The more traditional Triple Constraint metaphorLocked
The more traditional view of this triangle, of course, is that the Scope of work on a project is represented by the volume of the triangle (which in its ideal state is equilateral). As the initial project scope/size/complexity increases (pre signoff), the size of the triangle increases, but the shape should remain equilateral. The three points are labeled Cost, Failure tolerance and Time, and I think it would be a serious mistake to overlook the provision for quality. As you subsequently increase the Scope (post signoff), you must recognize an increase in one, two or all three of the other factors – that is, one or more of the vertices move further from the center of the picture. Cost or Time for the project must increase, and/or emphasis on Quality has to decrease.
Similarly, if you want to decrease Cost without decreasing the project Scope, you must be prepared to increase tolerance for Failure (i.e., decrease the quality effort) or increase the Time to market (or both).