A seating furniture support arrangement for a seat or a backrest, comprising: a first and a second load-carrying longitudinal beam, each being mountable to a seating furniture frame comprising one or more parts; an elastic sheet (16; 14) for providing support for one of said seat and said backrest; and a mechanism for stretching said elastic sheet between said first longitudinal beam and said second longitudinal beam, said mechanism comprising: said first longitudinal beam pivotable on said frame around a pivot axis, said elastic sheet (16; 14), with said seating furniture support arrangement in installed position, being connected to said first longitudinal beam offset to said pivot axis so that said first longitudinal beam pivoting with respect to said frame, in response to an application of an external force or torque, stretches said elastic sheet (16; 14), said external force or torque working against the tension force of said elastic sheet (16; 14) until a point of maximum tension is reached, past which point of maximum tension said tension force facilitates further pivoting of said first longitudinal beam, acting to keep said first longitudinal beam in place in a final position, characterized in that said elastic sheet (16; 14) is a first elastic sheet and said seating furniture support arrangement further comprises: a third longitudinal beam; and a second elastic sheet for providing support for the other of said seat and said backrest, said second elastic sheet also being connected to said first longitudinal beam of said mechanism, so that said pivoting of said first longitudinal beam also stretches said second elastic sheet between said first beam and said third longitudinal beam, and wherein said external force or torque works against the tension force of one of said first elastic sheet and said second elastic sheet up to said point of maximum tension, the other of said first elastic sheet and said second elastic sheet only being stretched with said pivoting having passed said point of maximum tension. a first and a second load-carrying longitudinal beam, each being mountable to a seating furniture frame comprising one or more parts; an elastic sheet (16; 14) for providing support for one of said seat and said backrest; and a mechanism for stretching said elastic sheet between said first longitudinal beam and said second longitudinal beam, said first longitudinal beam pivotable on said frame around a pivot axis, said elastic sheet (16; 14), with said seating furniture support arrangement in installed position, being connected to said first longitudinal beam offset to said pivot axis so that said first longitudinal beam pivoting with respect to said frame, in response to an application of an external force or torque, stretches said elastic sheet (16; 14), said external force or torque working against the tension force of said elastic sheet (16; 14) until a point of maximum tension is reached, past which point of maximum tension said tension force facilitates further pivoting of said first longitudinal beam, acting to keep said first longitudinal beam in place in a final position, characterized in that said elastic sheet (16; 14) is a first elastic sheet and said seating furniture support arrangement further comprises: a third longitudinal beam; and a second elastic sheet for providing support for the other of said seat and said backrest, said second elastic sheet also being connected to said first longitudinal beam of said mechanism, so that said pivoting of said first longitudinal beam also stretches said second elastic sheet between said first beam and said third longitudinal beam, and wherein said external force or torque works against the tension force of one of said first elastic sheet and said second elastic sheet up to said point of maximum tension, the other of said first elastic sheet and said second elastic sheet only being stretched with said pivoting having passed said point of maximum tension. a third longitudinal beam; and a second elastic sheet for providing support for the other of said seat and said backrest, said second elastic sheet also being connected to said first longitudinal beam of said mechanism, so that said pivoting of said first longitudinal beam also stretches said second elastic sheet between said first beam and said third longitudinal beam, and wherein said external force or torque works against the tension force of one of said first elastic sheet and said second elastic sheet up to said point of maximum tension, the other of said first elastic sheet and said second elastic sheet only being stretched with said pivoting having passed said point of maximum tension.