A coupling apparatus for a length measuring instrument is disclosed. The length measuring instrument includes a scale, which defines a division, and a scanning unit. The scale is connected to a first object and the scanning unit is connected to a second object movable relative to the first. The scanning unit is coupled to the second object by a coupling arrangement which is rigid in the measuring direction, and the scanning unit is guided on an auxiliary guide which functions independently of the guidance for the second object. The coupling arrangement includes a pivot joint with two degrees of rotational freedom along axes perpendicular to the measuring direction for compensation of direction errors, and a linear guide having two degrees of translatory freedom oriented perpendicularly to the measuring direction for correcting for non-parallelism between the auxiliary guide of the scanning unit and the guide of the second object.