If time, money, or what your project accomplished were unlimited, you wouldn't need to do project management. Unfortunately, most projects have a specific time limit, budget and scope.
It is this combination of elements (schedule, money and scope) that we refer to as the project triangle. Understanding the project triangle will allow you to make better choices when you need to make tradeoffs. If you adjust any one side of the triangle, the other two sides are affected.
For example, if you decide to adjust the project plan to
- Bring in the scheduled finish date, you might end up with increased costs and a decreased scope.
- Meet the project budget, the result might be a longer schedule and a decreased scope.
- Increase scope, your project might take more time and cost more money in the form of resources
Quality is at the center of the project triangle. Quality affects every side of the triangle, and any changes you make to any side of the triangle are likely to affect quality. Quality is not a factor of the triangle; it is a result of what you do with time, money, and scope.
For example, if you find you have additional time in your schedule, you might be able to increase scope by adding tasks and duration. With this extra time and scope, you can build a higher level of quality into the project and its deliverables.
Or if you need to cut costs to meet your budget, you might have to decrease scope by cutting tasks or reducing task durations. With decreased scope, there might be fewer opportunities to achieve a certain level of quality, so a lower quality results from the need to cut costs.
Jeff Atwood even carpented "the iron stool of software project management" out of this.