1. IN AN ACCORDION-TYPE OF MUSICAL INSTRUMENT, A PAIR OF MUTUALLY SPACED, OPPOSED WALLS FACING EACH OTHER AND DEFINING BETWEEN THEMSELVES AT LEAST PART OF A FIRST RESONANCE CHAMBER, ONE OF SAID WALLS BEING FORMED WITH A PLURALITY OF OPENINGS AND CARRYING A PLURALITY OF KEY-OPERATED VALVES FOR CONTROLLING SAID OPENINGS, RESPECTIVELY, AND THE OTHER OF SAID WALLS FORMING PART OF A SECOND RESONANCE CHAMBER AND BEING FORMED WITH A PLURALITY OF OPENINGS RESPECTIVELY ALIGNED WITH AT LEAST SOME OF SAID VALVES, AND CONTROL MEANS OPERATIVELY CONNECTED WITH SAID OTHER WALL FOR CONTROLLING SAID OPENINGS THEREOF.