In the literature, the bond shared by the three cycles is usually called the "bridge"; the shared carbon atoms are then the "bridgeheads". The notation ["x"."y"."z"]propellane means the member of the family whose rings have "x", "y", and "z" carbons, not counting the two bridgeheads; or "x" + 2, "y" + 2, and "z" + 2 carbons, counting them. The chemical formula is therefore C2+"x"+"y"+"z"H2("x"+"y"+"z"). 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, is the same substance as [3.2.1propellane. Therefore, it is customary to sort the indices in decreasing order, "x" ≥ "y" ≥ "z".