Abstract: With the invention a modular instrument system for knee joint prostheses is shown. A shaft (1), which can be provisorily but rotationally fixedly anchored along the anatomical axis (4) of the distal femur bone (2), terminates in the direction towards the joint with a collar (6). A parallel guide (7) is provided in the direction of a mechanical axis (9) from the collar (6) towards the interior of the shaft (1), which stands off by a guide angle a towards the medial to the shaft axis, which coincides with the anatomical axis (4). A coupling piece (10) with a projecting guide part (8) which is displaceably journalled in the parallel guide (7) serves as a reception for manipulation condyles (23) in order to test and determine the ideal position with respect to a tibia platform in the end positions of the articulation before resection blocks are positioned with the fixed coupling piece (10) for the final resection. The collar (6) is particularly advantageous in re-operations as a reference abutment (30).