A wide range of chemical clusters has been analyzed and categorized. In the process, it was discovered that main group elements obey the 4n series while the transition metals follow the 14n series. However, during subsequent analysis of the chemical clusters, it was discovered that the cluster valence electrons can also obey the 6n series for main group elements and 16n series for transition metal clusters. In this book, emphasis has been placed on a new type of series simply described as diagonal cluster series. A few examples to demonstrate the diagonal relationship of cluster valence electrons have been highlighted in Table 2.