_Harmonic Structure_. That form of tonal succession in which the tones of the melody follow rather persistently the structural outline of chords. _Major Pentatonic Scale_. That scale in which the constituent tones, if considered in upward sequence, would show the following arrangement of whole and whole-and-a-half-step intervals,--(whole) (whole) (whole-and-a-half) (whole) (whole-and-a-half). _Minor Pentatonic Scale_. That scale in which the constituent tones, if considered in upward sequence, would show the following arrangement of whole and whole-and-a-half step intervals,--(whole-and-a-half) (whole) (whole) (whole-and-a-half) (whole). The pentatonic scale is markedly primitive in character. It is known to have been in use anterior to the time of Guido d'Arezzo, which would give it a date prior to the beginning of the 11th century. [257] Rowbotham ascribes the invention of scales to those primitive musicians who, striving for greater variety in their one-toned chants, added first one newly-discovered tone, then another, and another. [258] The pentatonic scale might have resulted from such chanting. Most of the primitive peoples of the present day do not seem to feel or "hear mentally" the half step. If musicians of early days had this same failing, it was only natural for them to avoid that interval by eliminating from their songs one or the other of each couplet of tones which if sung would form a half step, thus their chants would be pentatonic. Not only do people in the primitive state fail to sense the half step, but also people in modern environment who have heard very infrequently this smallest interval of modern music. Inability to sense this interval may be better understood when we stop to consider that most of us find it unnatural and difficult to hear mentally the still smaller quarter-step interval or one of the even-yet-smaller sub-divisions of the octave which some peoples have come to recognize through cultivation, and have embodied in their music. This tendency to avoid the half step and develop along the line of pentatonic character is sometimes seen in our own children when they follow their natural bent in singing. It has been my observation that children with some musical creative ability, but unaccustomed to hearing modern music with its half steps, almost invariably hum their bits of improvised melody in the pentatonic scale. _Major Diatonic Scale_. That scale in which the constituent tones if considered in upward sequence would show the following arrangement of whole and half step intervals,--(whole) (whole) (half) (whole) (whole) (whole) (half). _Natural Minor Diatonic Scale_. That scale in which the constituent tones, if considered in upward sequence, would show the following arrangement of whole and half step intervals,--(whole) (half) (whole) (whole) (half) (whole) (whole).