From the tenth century to the twentieth, Western harmony has climbed the harmonic series. From the open-fifth chanting of Guido’s day to full acceptance of the triad in the later Middle Ages, from the extension of the triad to include sevenths in the Baroque era to the further accretions—ninths, elevenths, and thirteenths—in the nineteenth century, each addition to the vocabulary scaled the next higher rung on the overtone ladder.