Abstract: A music teaching and computation device in the form of a slide rule has a first display part on which are set out, by means of indicia the characteristic structures of five different modes of music. A second relatively movable display part co-operating with the first display part has indicia thereon representing notes of a musical keyboard, separated by spatial intervals in proportion to the musical intervals of pitch between the notes represented thereby. The first and second indicia can be brought into registry with each other, so as to compute the chords and scales in any of the presented given modes of music.