Abstract: A rotary disk calculator and educational aid is provided to assist in learning and understanding the relation between various musical keys on an instrument according to the circle of fifths system. A base plate member or disk is divided into twelve root key positions. An intermediate or second disk or template is concentrically mounted on the base disk. The structure of the second disk is alignable at various positions with each of the root key positions on the base plate. Indicators are formed on the second disk for ease of alignment. When aligned with a particular root key position, the second disk indicates the key signature (specific sharps or flats) that a piece of music has in that key; the number of whole or half steps between notes; notes for chords used in that key; and any special relationship linking certain of the notes.