The formation of clusters in nuclei is discussed and conditions to assess clusterization are formulated. By using these conditions a generalization of the Geiger-Nuttall law is deduced. This universal decay law (UDL) is a linear relation between the half-lives of the decaying clusters (including alpha particles) and the corresponding Q-values. The agreement between the UDL and available experimental data is excellent. This allows us to distinguish the role played by pairing collectivity in the clustering process.