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A medical marker device, comprising:

• a) a plurality of markers each having a planar marker surface, wherein the plurality of markers are each disposed on the marker device such that each marker of the plurality of markers displays mirror symmetry with respect to a mirror plane of symmetry which is common to each marker of the plurality of markers, and wherein the planar marker surfaces are disposed in the same orientation relative to a marker plane which is normal to the mirror plane of symmetry and to the surface normal of each of the marker surfaces;
• and
• b) an attaching unit for attaching the marker device to a marker holder having an elongate shape, wherein the attaching unit is disposed on the mirror plane of symmetry such that a longitudinal axis of the marker holder lies in the mirror plane of symmetry when the marker device holder is connected to the attaching unit;
• wherein the marker plane is tilted relative to the longitudinal axis of the marker holder at an angle of between 0° and 90°, excluding these values, when the marker device is attached to the marker holder, and wherein at least one of the markers is not disposed in the marker plane, and at least one of the markers is disposed in the marker plane.