One of these models became known as the concerted model, and the other model became known as a sequential model. Now, the first question you might be thinking is, why do we need two different models to describe a single phenomenon in nature? Well, as it turns out, each of these models alone does not correctly describe some aspect of the way that oxygen binds onto hemoglobin in a cooperative fashion. And where one model basically fails, the other model succeeds and vice versa. And that's exactly why we have to combine these two models to correctly describe the mechanism by which hemoglobin binds oxygen cooperatively. So let's begin with the concerted model.