During my Information Systems classes at Messiah and Temple the Project Triangle was always brought up as the element that would determine the scope in the various projects that we ended up working on.
In my most recent development project, I was hit with the triangle. The project triangle represents the three controlling factors of every project, out of which you can only choose two as being the necessary and most critical elements in order for the project to succeed.
The vertexes of the triangle represent the constraints of Cost, Features and Time. As an example, if you want a top-notch web site with all the bells and whistles and you’re also limited to a specific budget then time will be the factor that will change in order to still have the bells and whistles and stay on cost.