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. And this is a diagram that describes the concerted model. Now, the big point about the concerted model is hemoglobin can only exist in two states. It can either exist in the t state, the ten state, or it can exist in the r state, the relaxed state.