The concept of cone angle is most easily visualized with symmetrical ligands, e.g. PR. But the approach has been refined to include less symmetrical ligands of the type PRR′R″ as well as diphosphines. In such asymmetric cases, the substituent angles half angles, , are averaged and then doubled to find the total cone angle, θ'. In the case of diphosphines, the  of the backbone is approximated as half the chelate bite angle, assuming a bite angle of 74°, 85°, and 90° for diphosphines with methylene, ethylene, and propylene backbones, respectively. The Manz cone angle is often easier to compute than the Tolman cone angle: