Historically, the Solfeggio scale and its frequencies were used for teaching music and for sacred music such as Gregorian Chants. The scale was developed by Guido of Arezzo, an 11th-century Italian Benedictine monk who is regarded as a key figure in the history of Western music.
The Solfeggio scale was used in the teaching of singing during the Middle Ages, which was a critical period for the development of Western music. The system that Guido developed used a six-note ascending scale: ut, re, mi, fa, sol, and la. This sequence of tones is the root of where we get our modern “doe, ray, me, fa, so, la, tee, doe” – though you’ll notice that “ut” has been replaced with “do”, and a seventh note “si” (later changed to “ti”) was added.
The historical significance of Solfeggio Frequencies mostly lies in their influence on the development of music theory and education, particularly in teaching pitch and sight singing.
The reemergence of the Solfeggio Frequencies in the late 20th century, and their associated healing properties, is a modern addition to their historical significance. As of now, the scientific community generally views these claimed healing effects skeptically due to lack of empirical evidence. But the topic is popular within certain alternative healing and New Age circles.