Device for finding related accords, harmonies and keys - has rotary disc with windows to show all details for given key on marked base disc below it

摘要

The rotary disc assembly is intended as an aid in rapidly finding chords, harmonies and keys. It comprises a basic disc provided with appropriate markings, which is rotatable about its centre relative to a cover disc with windows. The arrangement is such that, when the discs are set to show a base key at the centre of an edge window, the main and secondary harmonic functions, the parallel minor key for a selected major key, the accord in basic position and first and second inversion, the seventh, major seventh and sixth to this accord can be read. Further details can be read off using a second disc with windows on the other side.