639 Hz is FA, described in the Solfeggio tradition as a tone for relationship and understanding, for warmth between people. It sits at the heart chakra, the centre of love and connection. The meaning is the tradition's, offered for contemplation. The Solfeggio names descend from the medieval hymn to St. John that Guido d'Arezzo used to teach singers: Ut, Re, Mi, Fa, Sol, La. In the 1970s Dr. Joseph Puleo reported recovering a six-tone set of pitches from a numeric reading of the Book of Numbers, and Leonard Horowitz carried the idea to a wide audience. Placing those six tones against the seven-chakra ladder is a later, devotional overlay, a way of hearing the scale as a climb through the body rather than a claim from the original tradition. Independent of any tradition, controlled clinical evidence that a specific frequency produces a specific health effect is limited and mixed. Auriviel presents these tunings as music and contemplative practice. This page is for general information and relaxation. Auriviel's frequency music is not medical advice, diagnosis, or treatment, and no health outcome is claimed. If you have a medical concern, please consult a qualified professional.