Why is there a semitone between 3 and 4 in the seven scales of music? Why not design it to increase in equal proportion? People who know a little about music, including me, will have such curiosity. There is no direct answer to the world of online search and the music history of China, so we can only analyze, contact and summarize it ourselves. Although I'm not a musician, I'd better talk about my superficial experience. In fact, there are many versions of scales in the world, but the most commonly used one, 1234567, is called "natural seven-tone scale". Therefore, the question to be asked is: in the natural chromatic scale 1234567, why are 3 and 4 arranged as chromatic scales instead of adding chromatic scales, that is, why not 1, 2, 3 and # 4? This question is natural and interesting! This is not a simple answer of seven numbers and a series of frequency numbers, but it is possible to get the answer by analyzing the history of music development. The natural seven-tone notes are produced in the long-term musical practice, which is the most frequently used among the twelve tones. Finally, the names are digitized instead of simply added. From the analysis of the phenomenon that China ancient pentatonic scale 12356 and western natural pentatonic scale 1234567 have the same pitch, it is no accident that 3 and 4 are semitones. In essence, the natural seven-tone scale is divided into two groups of whole tones: the former group is 123, and the latter group is 4567. There is a semitone between the two groups. Why are you doing this? China had wind instruments as early as the Spring and Autumn Period, and ancient Greece and the West had stringed instruments. It was discovered a long time ago that pure fourth degree (1-4, 2-5, 3-6) and pure fifth degree (1-5, 2-6, 3-7) and pure eighth degree are very harmonious and pleasing to the ear, and beautiful music cannot be separated from this. According to this correlation, China discovered the "three-point profit and loss law" very early in the Spring and Autumn Period: the pitch ratio of pure fourth degree is 3: 4, and the pitch ratio of pure fifth degree is 2: 3, from which the relative pitch of the pentatonic "Gong Shang Jiao Zheng Yu" is calculated, which is equivalent to modern 123-56. China ancient music and folk music are dominated by these five sounds, and it is difficult to see 4 and 7. Later, ancient Greece also discovered a similar "five-degree law", taking pure five degrees as a melody element. These two laws are only a few thousandths different from the "Twelve Average Laws" discovered by China in Ming Dynasty. People can't tell, but the accumulated error is still confusing. In order to correct the deviation, Greece has a "pure law". The core of these melodies are "pure fourth degree" and "pure fifth degree". In the current terminology, the number of intervals is "2.5" and "3.5" respectively, so it can be seen that the intervals before and after the scale must contain "semitones". "Pure fifth degree" is 1-5, 2-6, 3-7. From the perspective of music history, the two scales structure of 123-567 has actually been decided long ago. The seven-tone scale chooses 4 instead of #4 because 1-4 is the first pure fourth degree in the scale. Therefore, whether in China, Greece or the West, the existence and widespread use of "pure fourth degree" and "pure fifth degree" with semitones have laid the foundation of the natural scale, which inevitably leads to a "semitone" 4 between 3 and 5, and 7 and the next octave 1 are also semitones. Obviously, if the natural scale is a sextant scale with increasing proportion of 123456, it is impossible to directly express all the harmonic intervals such as "pure fourth degree" and "pure fifth degree" at the maximum convenience, and a large number of semitones must be used in music, which greatly reduces the practicability of the natural scale. (Original by Ye Hong)