When every valency in n monomer units is assigned an individual index number, a unique formula can be written for each non-cyclic n-mer by systematically listing the pairs of valencies forming the (n - 1) bonds. The chosen system allows the formulae to be partitioned into families and subfamilies and the number of ways of pairing the valencies in any set to be deduced. This leads to a new method of calculating the number of ways of forming noncyclic n-mers.