Abstract: A music rule is disclosed for assistance in the identification of chord, scale and mode structure with respect to the (ivory) key structure of a piano. The scale has a scale body accommodating an interior sliding rule. The interior sliding rule includes two areas for exposure to the user of the rule. The first exposure area--and preferably the lowest exposure area--is an analog of the piano keyboard. This analog of the rule provides for error-free registration of the rule to the keys of the piano. The second--and preferably higher exposure area--contains note indicia representative of the actual notes which the sliding scale overlies. The sliding scale fits to a rule body having first and second window areas. The first window body constantly exposes the keyboard analog of the piano keys. This enables registration of the rule in the entirety to the correct position on the piano keyboard.