All projects are carried out under certain constraints – traditionally, they are cost, time, and scope. These three important factors (commonly called "the triple constraint") are often represented as a triangle. Each constraint forms the vertices, with quality as the central theme.

• Projects must be delivered within cost.
• Projects must be delivered on time.
• Projects must meet the agreed scope – no more, no less.
• Projects must also meet customer quality requirements.

More recently, the triangle has given way to a project management diamond: Cost, time, scope, and quality are now the four vertices, with customer expectations as a central theme. No two customer expectations are the same, so you must ask specific questions about the customer's expectations.

Cost: All projects have a finite budget; the customer is willing to spend a certain amount of money for delivery of a new product or service. If you reduce the project's cost, you will either have to reduce its scope or increase its time.

Time (Schedule): As the saying goes, 'time is money', a commodity that slips away too easily. Projects have a deadline date for delivery. When you reduce the project's time, you will either have to increase its cost or reduce its scope.

Scope: Many projects fail on this constraint because the scope of the project is either not fully defined or understood from the start.

When you increase a project's scope, you will either have to increase its cost or time. Once a customer asks you to complete a project, the person will state what is important; for example, the project must cost no more than £80k, be delivered by a particular date, or contain certain features. The triple constraint is about balancing each constraint to reach a successful conclusion. Another key factor of any project, is the control of that change.