To evaluate whether any distinguishing behavior of the studied systems can be inferred from the map, we define the line connecting the two minima, ⃗ d, as a 'line of concertedness', in analogy to the diagonal of the energy-based square scheme. While concerted vs. sequential PT and ET are binary categories in the energy-based square scheme and coupling is ill-defined, our map in real space dimensions offers a more concrete and gradated view of these concepts based on the relative orientation of the ⃗ p, ⃗ q and ⃗ d vectors. We propose the degree of coupling is defined by the angles between ⃗ p and ⃗ q with respect to ⃗ d. This is expressed mathematically as the dot product of ⃗ p and ⃗ q, or equivalently as the cosine of the sum of the angles, see Figure . Coupled proton-electron transfer (CPET) then describes the situation in which movement along both the PC and ET directions is needed until the product minimum is reached. In a more strongly coupled system, the angle between the PC and ET dimensions decreases and eventually becomes zero (at which point the coordinates are indistinguishable), whereas in a minimally coupled system, they are found at an angle of 90°. We emphasize that whenever the two directions of movement are not orthogonal, any movement along the proton coordinate implies some movement along the ET dimension and vice versa.