Abstract: The ruler for composing or decomposing the chords for polyphonic musical instruments is characterized in that it comprises: ruler slides (A,B,C,D,E,F,P,Q) movable with respect to each other in a given direction and each provided with inscriptions relative to the notes of the chromatic scale, respectively assigned to the fundamental, the third, the fifth, the sixth, the seven the etc. . . . of chord--a cursor (K,N) relatively movable in said given direction with respect to the ruler slides, said cursor being with a marking (L,N5) positioned to make evident one of the notes of the chromatic scale of each of the slides: one element (H) integral with the slide (A,P) assigned to evident the inscriptions relative to the musical chord searched for the invention applies to a chord ruler for polyphonic musical instrument.