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A chair, in particular an office chair, comprising: a base structure, a seat support structure, articulated to the base structure around a transverse axis, an adjustable elastic device to apply an elastic force between the base structure and the seat support structure, a manually operated adjustment

A chair, in particular an office chair, comprising: a base structure, a seat support structure, articulated to the base structure around a transverse axis, an adjustable elastic device to apply an elastic force between the base structure and the seat support structure, a manually operated adjustment device for varying the distance between said adjustable elastic device and said transverse axis.

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1. A chair, in particular an office chair, comprising:a base structure, a seat support structure, articulated to the base structure around a transverse axis, an adjustable elastic device to apply an elastic force between the base structure and the seat support structure, an adjustment device which c

1. A chair, in particular an office chair, comprising:a base structure, a seat support structure, articulated to the base structure around a transverse axis, an adjustable elastic device to apply an elastic force between the base structure and the seat support structure, an adjustment device which can be operated manually to vary a distance between said adjustable elastic device and said transverse axis, wherein, in a resting position of the seat, said adjustable elastic device does not apply force to the seat support structure, so that the operation of the said adjustment device in said resting position of the seat is carried out without any elastic reaction force acting on the adjustable elastic device, and wherein the adjustable elastic device comprises a support, movable relative to the base structure along a longitudinal direction and bearing at least an elastic element associated with at least a member for applying a load. 2. A chair as claimed in claim 1, comprising a stationary elastic device to apply an elastic force between the base structure and the seat support structure, biasing the seat support structure towards said resting position.3. A chair as claimed in claim 1, wherein said resting position of the seat said member for applying the load is not in contact with the seat support structure.4. A chair as claimed in claim 3, wherein the manually operated adjustment device comprises a transverse rod rotatable relative to the base structure around a transverse axis and associated with a transmission mechanism to actuate the translation of said support in the longitudinal direction as a function of the rotation of said rod.5. A chair as claimed in claim 4, wherein said rod extends coaxially to the axis of articulation between the base structure and the seat support structure.6. A chair as claimed in claim 1, comprising a backrest support structure articulated to the base structure around a second transverse axis parallel to the axis of articulation between the seat support structure and the base structure.7. A chair as claimed in claim 6, comprising at least one connecting rod with its ends articulated respectively to the seat support structure and to the backrest support structure, so that oscillating motions of the seat support structure and of the backrest support structure about the respective axes are mutually synchronized.8. A chair as claimed in claim 1, comprising a locking device to lock the seat in a position selected by a user.9. A chair as claimed in claim 8, wherein said locking device comprises a locking pivot borne by the seat support structure and movable in a vertical direction relative to the base structure, the locking device comprising a locking lever destined to co-operate with a plurality of annular grooves formed on said locking pivot.10. A chair as claimed in claim 9, wherein said locking lever is connected to a longitudinal transmission rod movable in longitudinal direction between a locking position and an unlocking position and associated with a rotatable actuation sleeve, operated manually by the user.11. A chair as claimed in claim 1, comprising a device for adjusting a height of the base structure, which includes a rotatable sleeve associated to a longitudinal transmission rod co-operating with a command member of a gas spring.12. A chair as claimed in claim 1, comprising a seat movable in the longitudinal direction relative to the seat support structure.13. A chair as claimed in claim 12, wherein longitudinal motion of the seat is synchronized with oscillating motion of the seat support structure around said transverse axis.14. A chair, in particular an office chair, comprising:a base structure, a seat support structure, articulated to the base structure around a transverse axis, an adjustable elastic device to apply an elastic force between the base structure and the seat support structure, an adjustment device which can be operated manually to vary a distance between said adjustable elastic device and said transverse axis, a seat movable in the longitudinal direction relative to the seat support structure, wherein the longitudinal motion of the seat is synchronized with the oscillating motion of the seat support structure around said transverse axis, and wherein the chair further comprises a backrest support structure including a pair of arms provided with respective appendages which engage respective longitudinal shoes fastened to the seat and movable in the longitudinal direction relative to the seat support structure.