Jig for determining a patient-adapted implant size of the femoral implant of a knee endoprosthesis, wherein the jig comprises the following: a main body (2, 4), a probe part displaceable relative to the main body (2, 4) along a measuring direction, wherein the probe part has a probe arm with a probe tip for bearing on an anterior reference point of the distal end of the patient's femur, two bearing pieces (11, 12) for bearing on and referencing the medial posterior and the lateral posterior condyle, respectively, of the distal end of the patient's femur, and at least one scale (23, 24) and an associated indicator (21, 22) for indicating a position of the probe part in the measuring direction relative to at least one of the bearing pieces (11, 12) for specification of a suitable implant size, wherein the bearing pieces (11, 12) are movable relative to the main body (2, 4) in such a way that a distance of the probe tip to the bearing pieces (11, 12), projected onto the measuring direction, is adjustable for the bearing pieces (11, 12) in such a way that this distance is different with respect to the two bearing pieces (11, 12), and wherein each bearing piece (11, 12) is provided with its own scale (23, 24) and an associated indicator (21, 22) for indicating a position of the probe part in the measuring direction relative to the respective bearing piece (11, 12) and for the resulting specification of a suitable implant size, characterized by a coupling between the bearing pieces (11, 12), which coupling, in the case of a relative movement of one bearing piece (11, 12) relative to the main body (2, 4), also causes a relative movement of the other bearing piece (12, 11) relative to the main body (2, 4). a main body (2, 4), a probe part displaceable relative to the main body (2, 4) along a measuring direction, wherein the probe part has a probe arm with a probe tip for bearing on an anterior reference point of the distal end of the patient's femur, two bearing pieces (11, 12) for bearing on and referencing the medial posterior and the lateral posterior condyle, respectively, of the distal end of the patient's femur, and at least one scale (23, 24) and an associated indicator (21, 22) for indicating a position of the probe part in the measuring direction relative to at least one of the bearing pieces (11, 12) for specification of a suitable implant size, wherein the bearing pieces (11, 12) are movable relative to the main body (2, 4) in such a way that a distance of the probe tip to the bearing pieces (11, 12), projected onto the measuring direction, is adjustable for the bearing pieces (11, 12) in such a way that this distance is different with respect to the two bearing pieces (11, 12), and wherein each bearing piece (11, 12) is provided with its own scale (23, 24) and an associated indicator (21, 22) for indicating a position of the probe part in the measuring direction relative to the respective bearing piece (11, 12) and for the resulting specification of a suitable implant size, characterized by a coupling between the bearing pieces (11, 12), which coupling, in the case of a relative movement of one bearing piece (11, 12) relative to the main body (2, 4), also causes a relative movement of the other bearing piece (12, 11) relative to the main body (2, 4).