The triangle consisting of time, cost and quality on its sides is referred as scope triangle. This is very important metrics of measurement of project status. It is also known as Quality triangle.
The time parameter refers to time duration to deliver the project. The cost parameter refers to money available for the project. The quality parameter refers to quality standard need to be met during the execution of the project. The fig-1 scope triangle depicts how these three aspects form a triangle.
Fig-2 mentions normal and healthy project condition where in one of the factors is fixed and other two factors can vary. In the figure two cases are mentioned. In case-1 time is kept fixed and cost/quality are varied while in case-2 quality is kept fixed and time/cost is varied. In these both cases triangle is complete and hence project is said to be under control.
In fig-3 depicts a case wherein two factors are fixed(cost and quality) and third one(i.e. time) is varied. In this case time is less than the other two factors and hence triangle does not complete by itself. This gap between time and quality is recognized as scope creep.