Any of several sets of three chemically similar elements, the atomic weight of one of which is approximately equal to the mean of the atomic weights of the other two are known as traids. For example, chlorine-bromine-iodine, calcium-strontium-barium, and sulfur-selenium-tellurium. This was noted by the German chemist J.W. Döbereiner. The triad was the earliest atomic-weight classification of the elements.