144 THE NATURE OF MUSIC 



At each intersection four components of V9 form 

 the tetrachord of three consecutive whole steps known 

 as the tritonus, as follows : — 



•^ 1 9 3 



fa — sol — la — ti 

 — ^^ 



A few of the dissonant chords formed by combining 

 these tones are next given in close and open positions. 



9 



3 3 



1 -^ 9 1 9 3 9 9 '^ 



119 19 3 11 



7 -g?- 



i 



i 



s?^ f > i-g^ 



25^ 



-^^ ^- 



^^ TSr 



V, V9 



Here observe in passing that the tri tonus is the 

 only tetrachord whose four tones are components of 

 one harmony and together form a simple chord of 

 the dissonant type. 



A septonate is named by its central tone: thus the 

 above septonate is the Tonic-septonate. A diatonic 

 scale of eight tones is named by its terminal: the 

 above octonal scale is likewise a Tonic-scale. Each 

 of the seven diatonics may appear as a septonal 

 centre of two conjunct tetrachords and the octonal 

 terminal of two disjunct tetrachords, and each such 

 scale like that of the Tonic is named by its septonal 

 centre and octonal terminal. Thus sol the Dominant, 

 fa the Subdominant, mi the Mediant, la the Sub- 

 mediant, re the Supertonic, ti the Subtonic, each of 

 these may be the septonal centre and the octonal 

 terminal and, as these syllables and names imply, all 



