Metrical structure in music is thus, obviously enough, fairly analogous with metrical structure in verse, with its grouping of syllables into feet, of feet into verses, and of verses into couplets or stanzas. When we pass to the second sort of musical structure, however, which we may call tonal or harmonic structure, the parallel analogy with poetic rhyme is much less satisfactory. It is true that harmony and rhyme both act by presenting similar sounds at given points in the series of impressions; but harmony is a far more subtle, various, and potent organizing agent than rhyme. Harmony depends on the fact that the tones, or pitch elements, used in music, can be distinguished into unrestful and restful, or into transitional and final, just as the metrical or time-elements are. In primitive music, in which but one tone sounded at a time, the matter was almost absurdly simple: high notes were unrestful, because they involved muscular tension;[26] low notes were restful, because they meant relaxation of vocal effort. Consequently, a descent of the voice meant a transition to a goal, and songs were divided off into sections by successive falls of the voice or cadences. The word “cadence,” so important in musical terminology, preserves in itself the record of this phase of musical growth; from the Latin _cado_, to fall, it means primarily a sinking or lapsing, and hence, in general, a coming to rest. As soon as two or more melodies were sounded together, however, the sense of rest following activity, the universal generator of design in a temporal series of impressions, could be produced in a far more subtle way. It could be produced by making the melodies pass through an inharmonious or dissonant chord or series of chords, to a harmonious one. As soon as dissonance came into general use, in other words, the sense of unrest, of impulsion toward something else, of progressive movement, that it imparted to music, was so potent that cadences could be made upward as well as downward; whenever dissonance resolved into consonance the effect of cadence ensued. And as dissonances are of all conceivable degrees of harshness, cadences could be made of any desired degree of finality. Moreover, as the tonal material of music grew more and more systematized, the feeling of key sprang up in men’s minds; all music was felt to be in a certain key, that is, grouped about a certain tone, the centre and goal of all the others; and then cadences came to have even greater variety in the degree of finality they seemed to assert, dependent not only on the strength of the dissonances they followed, but also on the remoteness or nearness of their final chord to the key-note of the piece. All this meant greater and greater resources for building up music into complex and yet perfectly definite organisms; and as harmonic form constantly interacted more and more subtly with metrical form the capacities of design became practically infinite.