The PM literature refers to the triple constraint of scope, time and cost as the "iron triangle" to help visualize the interlocked dependencies between these constraints. Imagine a triangle made of iron, with each corner of the triangle corresponding to the scope, time and cost, respectively, while the iron core represents the quality of the result produced by a given project. If you try to stretch any corner, you're bound to break the quality at the core.
Im not sure I buy this metaphor. In my view both time and cost are resources, together with other, like people, skill-set, etc. Each of these have a bearing to the outcome of the project. You may be tackling an impossible problem, so whatever resources you throw at it you're still far from meeting the goals of the project, as defined by its scope. That is, until you come across the mix of resources that makes the impossible possible.
So sow about a silk string instead of the iron triangle ? At one end is the scope, at the other are the resources. The string itself is the quality. If you pull too hard at one end it's bound to break.