In zijn boek Management 3.0 beschrijft Jurgen Appelo zijn eigen variant op de bekende 'duivelsdriehoek'.

The problem with quality is that it is often simply assumed by everyone. This is exemplified by the well-know triangle of constraints, or project management triangle, which lists three important constraints (scope, cost, and schedule), but not quality. Customers just assume they will get quality products, and managers assume that employees know how to build them. (...) [Jurgen Appelo prefers] to depict these choices in [his] favorite adaption of the iron triangle, where quality is added to turn the triangle into a square. The idea of the square is that changing one corner in one direction has a similar effect on either of the two adjacent corners, or else a reverse effect on the opposite corner. For example: Increased functionality means more resources or an increased time schedule, or else a lower quality. And loss of resources leads to less functionality or decreased quality, or else an increased schedule.

Bron: Management 3.0: Leading Agile Developers, Developing Agile Leaders, Jurgen Appelo