 The minimum value for "x", "y", and "z" is 1, meaning a 3-carbon ring.There is no structural ordering between the rings, so, for example, [1.3.2]propellane is the same substance as [3.2.1]propellane.Therefore, it is customary to sort the indices in decreasing order, "x" ≥ "y" ≥ "z".In the propellanes with small cycles, such as [1.1.1]propellane or [2.2.2]propellane, the two carbons at the ends of the axial bond will be highly strained, and their bonds may even assume an inverted tetrahedral geometry.