Ideally, a project manager would like to complete a project on time, within budget, according to scope and quality requirements and achieve customer satisfaction. In a perfect project environment, this would be relatively easy to achieve if a well-thought-out plan was in place and the team was ready to handle any possible change or risk event that might possibly occur. In actual practice projects seldom progress exactly as planned. Customers request changes, the environment doesn't cooperate, people make mistakes, estimates are too optimistic, items are omitted or overlooked, conflicts occur, and team members are changed more frequently than we would like (churn or turnover of project teams can create rework, impede progress, and create conflicts). This "true" project environment drives the project manager and team to continually assess the project and, working within the identified and generally inflexible constraints, trade-offs (the giving up of one item to satisfy another need) must be considered. Project managers must be willing to analyze situations and decide on actions in a give-and-take project environment.
The time, cost, performance triangle, known as the Triple Constraint, is the "magic combination" that provides the basis for project trade-off decisions and keeps the project manager continuously pursuing a balance of these elements to achieve project success.