The basic premise of a productivity triangle is simply this:
if you want quality work at low cost, be prepared to spend a little extra time
if you want quality work, but you want it very quickly, it will cost you a pretty packet
and finally
if you want it quickly and cheap as well, the quality will, most certainly, suffer.
The shaded area in the diagram here is bT Square Peg's preferred zone for most projects. Quality gets first priority, cost is close behind and time; well that's important too but experience has shown that haste really does make waste!
Actually, if you want to be really accurate, there is a fourth factor in this equation - the scope of work.
In the triangle above, we've assumed the scope of work as constant. This is hardly the case in reality and, in my experience, the scope invariably increases after the work has begun (and after the estimates are made!).
This factor can have a major impact on the schedule as well as the budget so, if the scope of work is increased, one must understand that - to maintain the same quality - the cost as well as the time to completion will increase proportionately.