Carl here. Recently I had the opportunity to share a model called the "triangle of triple-constraints" or more simply the "project triangle" with some bright people I know. This is a model familiar to many project managers, and I have found it's a useful framing tool when discussing that elusive notion of "quality."
Triangulating on an idea
I wrote an appendix called "A Short Course in Project Management" for the 2000 edition of the Project Step by Step, and we've kept it in the subsequent editions with minor changes. You can find the excerpted appendix here on Office Online. You can also read the "Project Management Triangle" section of this Wikipedia entry.
Very briefly, the project triangle model as a way of visualizing the three critical ingredients of a given project: time, cost and scope. Here's one simple graphic representation:
A project manager's intent is two-fold: first, quantify and define these ingredients to the point that they can be represented in planning tools like Microsoft Project, and then later respond to variance in time, cost or scope once work on the project has gotten under way.
The project triangle is a great instructional tool--pretty much everybody gets it with just a short explanation, and it can often prompt lots of excellent follow-up questions. For example, when I bring up the notion of quantifying resource capacity, that can lead to more pointed questions about how an organization measures people's working time and allocation across projects and other types of work.