Way back in the 11th century, a Benedictine monk named Guido d’Arezzo introduced the musical scale that we now know as the Solfeggio frequencies – though some argue they’re even older than that. The monks used these original six Solfeggio notes – 396, 417, 528, 639, 741, and 852 – in their Gregorian chants.
Source: 6 tones of the ancient Solfeggio Scale – Google Search