The triple constraints of time, cost, and scope bounding quality are really a minimum of 4 constraints in my mind, since changing the quality expectations can affect all 3.
Some models include quality, risk, and benefit as a separate triangle overlaying the other 3. Yet others include benefit as part of scope and instead include resources as both having enough of the right resources can certainly limit what is possible.
In models where quality is one of the constraints, sometimes it shows quality also in the middle which doesn't make sense to me. I prefer the Venn diagram depiction of all the constraints as overlapping circles with success the intersection of all the constraints together. Saving Changes...
One more to add to Keith's list are stakeholder expectations as those can constrain what the team could or could not do, and in some cases that constraint is the most important one to meet.
