In Chapter 7, Schaeffner follows, step by step, the action of the different solid bodies that resonate inside cavities. The progression of his argument is clear. Building on the foundations laid in Chapters 1 to 3, he notes the introduction of “instruments superimposed onto their antecedents”—the mouth onto the Jew’s harp or the body of the violin onto the strings, for example. This leads him to a typology of “solid bodies: rigid, flexible or tensionable.” At this juncture, he is able to cast an eye over the instrument classifications proposed by his predecessors Victor Mahillon, Erich von Hornbostel and Curt Sachs. He pays tribute to the importance of these classifications—today, museology still follows the now-standard division into idiophones, membranophones, chordophones and aerophones, which Schaeffner mentions—but having adopted a resolutely ethnological point of view, he criticizes Hornbostel, Sachs and others for not having taken into account the means of playing: here again, he insists on the importance of resonance.