For a nonlinear ABn system there are 𝑁 = 𝑛 + 1 atoms and hence the internal degrees of freedom number 3𝑛 -3. For ABn, there are 𝑛 A-B i bonds thus the 3𝑛 -3 degrees of freedom can be partitioned as 𝑛 A-B i bond stretching degrees of freedom and 2𝑛 -3 "angle changing" degrees of freedom. Fig. summarises this partitioning for nonlinear and linear ABn systems. Coordination geometries of ABn are frequently described and named with reference to the "coordination polyhedron" (more formally, the "convex hull") defined by the nuclear positions of the A-atom and all B-atoms . This picture works for the purposes of chemical nomenclature where only an approximate geometry need be discussed and models typically idealise the geometry to some high-symmetry form. This "idealisation process" can be viewed as comprising two parts: i) treating all A-B i bond lengths as equal, and ii) changing the interior angles B i -A-B j angles as appropriate. It follows that the primary focus of defining a coordination geometry's configuration depends upon the set of interior angles at the A-atom. A simple demonstrative example illustrates this. In Fig. is shown a trans-Pt complex where all four angles at Pt are precisely 90° due to symmetry but the (DFT-calculated ) bond lengths are notably different as indicated. The rhombus shaped convex hull of the coordination sphere for this complex is indicated by dotted lines in Fig. .