As a slightly pedantic aside, I prefer Kent Beck's observation that the triangle is time, scope, resources, with quality independent of those three. Of course,

As a slightly pedantic aside, I prefer Kent Beck's observation that the triangle is time, scope, resources, with quality independent of those three. Of course, quality can be sacrificed in the short term to improve time or scope, but the impact is so gruesomely negative to their long-term prospects that it should simply never be done. Resources as a leg also lines up with the observation from The Mythical Man-Month that adding resources to a late project makes it later. (I have experienced this directly.) So resources are rarely mutable to a significant degree; you have the team you have and the budget you have - and you really do not want to see what happens when you suddenly get an unlimited budget! So I tend to phrase it as every project is either scope-bound or schedule-bound. One of those can flex - which one? If you can't flex either, you will probably fail.