Standard musical tuning, specifically in the context of Western music, is typically based on a system known as equal temperament. In this system, the octave (the interval between one musical pitch and another with double its frequency) is divided into 12 equal parts, known as semitones. The frequency ratio between each note in this 12-note scale is the twelfth root of two (approximately 1.0595).
For example, if we start with the note A4, which is commonly tuned to 440 Hz, the next higher A (A5) would be 880 Hz, and the 12 semitones between these two A’s (representing the notes A#, B, C, C#, D, D#, E, F, F#, G, G#, and the next A) would each have a frequency that’s approximately 1.0595 times the frequency of the previous note.
In contrast, the Solfeggio Frequencies are not based on the equal-tempered scale. Instead, they represent a series of frequencies (396 Hz, 417 Hz, 528 Hz, 639 Hz, 741 Hz, and 852 Hz) that are said to have specific healing or transformative properties. When used individually these frequencies don’t align with the notes of the equal-tempered scale, nor do they represent equally-spaced divisions of the octave. They appear to be based more on numerological considerations than on traditional music theory.
That said, when re-tunning a piece of music to a Solfeggio Frequency tone, the entire music scale is re-tuned around that tone. For example, when retuning the universal 440 Hz A4 tone to 441.8 Hz, G#4 is the Solfeggio Frequency 417 Hz. We can retune the universal 440 Hz A4 to different frequencies to include the Solfeggio Frequencies in the scale.