A coupling device for coupling a rod to a bone anchor comprising a receiving part having a channel for receiving a rod and having a passage defining a central axis and an accommodation space for a head of the bone anchor, the receiving part comprising an outer surface with an engagement structure configured to be engaged by an instrument; and a pressure member configured to be arranged at least partially in the accommodation space, the pressure member comprising a flexible portion defining a hollow interior to accommodate and clamp the head therein and an outer surface with an engagement structure configured to be engaged by the instrument, wherein the pressure member can assume a first position in which the pressure member engages an inner wall portion of the passage such that the flexible portion is compressed to clamp or lock an inserted head with a first locking force and a second position in which an inserted head is clamped or locked with a second locking force smaller than the first locking force or is freely pivotable; wherein the pressure member is movable from the first position towards the second position by engagement with the instrument; wherein the engagement structure of the receiving part and the engagement structure of the pressure member are offset relative to each other in the axial direction defined by the central axis when the pressure member is in the first position such that when the engagement structure of the receiving part is first engaged by the instrument and the engagement structure of the pressure member is subsequently engaged by the instrument, the pressure member is moved from the first position into the second position and wherein an outer surface of the flexible portion of the pressure member comprises a narrowing section and wherein the narrowing section is tapered and extends along substantially the whole length of the flexible portion. a receiving part having a channel for receiving a rod and having a passage defining a central axis and an accommodation space for a head of the bone anchor, the receiving part comprising an outer surface with an engagement structure configured to be engaged by an instrument; and a pressure member configured to be arranged at least partially in the accommodation space, the pressure member comprising a flexible portion defining a hollow interior to accommodate and clamp the head therein and an outer surface with an engagement structure configured to be engaged by the instrument, wherein the pressure member can assume a first position in which the pressure member engages an inner wall portion of the passage such that the flexible portion is compressed to clamp or lock an inserted head with a first locking force and a second position in which an inserted head is clamped or locked with a second locking force smaller than the first locking force or is freely pivotable; wherein the pressure member is movable from the first position towards the second position by engagement with the instrument; wherein the engagement structure of the receiving part and the engagement structure of the pressure member are offset relative to each other in the axial direction defined by the central axis when the pressure member is in the first position such that when the engagement structure of the receiving part is first engaged by the instrument and the engagement structure of the pressure member is subsequently engaged by the instrument, the pressure member is moved from the first position into the second position and wherein an outer surface of the flexible portion of the pressure member comprises a narrowing section and wherein the narrowing section is tapered and extends along substantially the whole length of the flexible portion.