We computed similarity matrices using all chemicals available for every year in the period 1800-2021. These matrices are subsequently used in various ways to represent and ultimately to gain insight into different aspects of the evolution of the periodic system, as shown in Fig. . Aiming at contrasting the periodic systems of different years, we encode periodic systems in low dimensional representations. Research on these type of representations, and in particular in optimal sequences of elements, began in the 1980s when Pettifor introduced the first sequence and used it to visualise and estimate properties of binary inorganic compounds . More recently, further approaches to the calculation of optimal sequences and extended applications have been developed .