Abstract: This invention consists of a device for composing, decomposing, retransposing and retranscribing all chords and scales contained in music. The device in question comprises two superimposed disks of different diameters, joined at their center by a pin enabling the disks to rotate independently. The twelve musical notes of the diatonic scale are written on the lower disk with the larger diameter. Intervals, interval qualifications, degrees of intervals and various indexed scales are written on the upper disk with the smaller diameter. Whatever the composition, decomposition, retransposition and retranscription information required for a given chord or scale, the user, by rotating the disks, matches the chosen note with the "FUNDAMENTAL" interval, on which all chord or scale building is based. This allows him to see the position, degree, interval and qualification of the other notes of the chord or scale. This device may be used for all diatonic, monophonic and polyphonic instruments.