The relationship tone of the Solfeggio scale
639 Hz carries the syllable "Fa" in the Solfeggio sequence and sits at the center of the nine-tone set. Sound-healing practice assigns it to the heart chakra (Anahata) and uses it with intentions around empathy, communication, and repairing relationships - both with others and with oneself.
The frequency assignments themselves date to the 1990s numerology of Joseph Puleo rather than to ancient practice, but the human association between heart-centered emotion and warm mid-high tones is much older - it is the register of the singing voice at emotional peak, roughly between E5 and F5 (639 Hz is about 22 cents sharp of E5 in standard tuning, and close to D#5/Eb5 territory in feel).
Listening suggestions
A common practice is pairing 639 Hz with loving-kindness (metta) meditation: play the tone at low volume and direct attention toward a person or relationship with each breath cycle. Others play it as an ambient drone during difficult conversations preparation, journaling about relationships, or couples yoga.
As with all single-frequency work, the evidence-backed mechanism is calm and focused attention rather than anything specific to 639 Hz. The tone is a clean, singing pitch - bright enough to hold attention, low enough to stay unfatiguing at soft volume. Sine gives the purest drone; sawtooth is too harsh for most meditation uses at this pitch.