This has been called a "diagonal" PR, or sometimes a diagonal length scale, although the extracule coordinate lies along the anti-diagonal direction in the matrix representation Ω. A schematic view of how to interpret that matrix is provided in Fig. . As indicated in that figure, the CT indices Ω A→B (and the two-dimensional matrix Ω formed from them) are essentially a coarse-graining of the transition density T (r elec , r hole ) expressed in particle/hole coordinates. This is discussed further in Section 5.