In my interview with Jorge Arango (just posted this weekend), among other topics, I described the missing third diamond from the Design Council’s Double Diamond model.
The third, missing, diamond is that of strategy, where project criteria, metrics, and expectations are set. Often designers aren’t part of the decision-making at this stage, even though most designers have the right skills for strategy, Until designers can access this phase, they will always be subject to the wrong strategy and decisions and, thereby, so will customers.
There is an old adage: “Instead of designing the thing right, we need to focus on designing the right thing.” This third diamond, strategy, is where either the right or wrong thing is decided. You can’t fix this decision afterward in more client and organizational contexts.
This is described more in A Whole New Strategy