TRIAD

in music, a chord of three tones, consisting of a root with its third and fifth and constituting the harmonic basis of tonal music. See chord. in chemistry, 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. Such triadsincluding chlorine-bromine-iodine, calcium-strontium-barium, and sulfur-selenium-telluriumwere noted by the German chemist J.W. Dbereiner between 1817 and 1829. The triad was the earliest atomic-weight classification of the elements.