Accordion of chromatic or diatonic type comprising: - a first keyboard, usually referred to as right-hand or melody manual (6,29,31,33), arranged on one side of the instrument and supported by a first box connected to one of the ends of the bellows, said keyboard enabling the accordionist to play the melody with one hand, being formed either by piano-style keyboard keys comprising white keys and black keys or by a button-style layout with buttons arranged side by side on several rows which may be staggered or not with respect to one another, - a second keyboard, usually referred to as left-hand or accompaniment manual (7,30,32,34), arranged on the other side of the instrument and supported by a second box connected to the second end of the bellows, said keyboard enabling the accordionist to play bass notes and chords, with his other hand, to provide the accompaniment and rhythmics of the melody, - a bellows (5,8,9,10,11,12,13,14,15,16,17) formed by a juxtaposition of several inner and outer pleats each having the form of a closed frame, which can be designated by the general term of pleats, arranged side by side and joined to one another by their adjacent movable walls so that each of the two movable walls of an inner pleat forms one of the two movable walls of the adjacent outer pleat and vice versa, - each of the inner pleats is formed by two movable walls joined to one another by a common edge thereby constituting a dihedron open in the direction of the outside of the bellows, the angle of said dihedron varying from substantially zero degrees when the bellows is completely folded to about 90° or even more when the bellows is fully deployed, - and each of the outer pleats is formed by two movable walls joined to one another by a common edge thereby constituting a dihedron open in the direction of the inside of the bellows, the angle of said dihedron varying from substantially zero degrees when the bellows is completely folded to about 90° or even more when the bellows is fully deployed, characterized in that the closed peripheral circumference of at least certain of the pleats foming the bellows (5,8,9,10,11,12,13,14,15,16,17) forms a polygon of irregular shape in so far as the sides of said polygon are not all equal to one another nor are they all equal two by two. - a first keyboard, usually referred to as right-hand or melody manual (6,29,31,33), arranged on one side of the instrument and supported by a first box connected to one of the ends of the bellows, said keyboard enabling the accordionist to play the melody with one hand, being formed either by piano-style keyboard keys comprising white keys and black keys or by a button-style layout with buttons arranged side by side on several rows which may be staggered or not with respect to one another, - a second keyboard, usually referred to as left-hand or accompaniment manual (7,30,32,34), arranged on the other side of the instrument and supported by a second box connected to the second end of the bellows, said keyboard enabling the accordionist to play bass notes and chords, with his other hand, to provide the accompaniment and rhythmics of the melody, - a bellows (5,8,9,10,11,12,13,14,15,16,17) formed by a juxtaposition of several inner and outer pleats each having the form of a closed frame, which can be designated by the general term of pleats, arranged side by side and joined to one another by their adjacent movable walls so that each of the two movable walls of an inner pleat forms one of the two movable walls of the adjacent outer pleat and vice versa, - each of the inner pleats is formed by two movable walls joined to one another by a common edge thereby constituting a dihedron open in the direction of the outside of the bellows, the angle of said dihedron varying from substantially zero degrees when the bellows is completely folded to about 90° or even more when the bellows is fully deployed, - and each of the outer pleats is formed by two movable walls joined to one another by a common edge thereby constituting a dihedron open in the direction of the inside of the bellows, the angle of said dihedron varying from substantially zero degrees when the bellows is completely folded to about 90° or even more when the bellows is fully deployed,