https://www.musical-u.com/learn/who-moved-the-tonic-part-1-hearing-key-changes/
The tonic gives you most of the information you need to play and/or transcribe a melody without written music. It tells you the key, the scale, the accidentals, the potential chord progressions and more. https://www.musical-u.com/learn/who-moved-the-tonic-part-1-hearing-key-changes/