At the European XFEL, two hard X-ray self-amplified spontaneous emission (SASE) undulators and one soft X-ray SASE undulator will be used for producing high brightness laser radiation. The hard X-ray undulators SASE1 and SASE2 will consist of 35 undulator segments, each 5 m long, separated by 1.1 m long intersections for e-beam steering, focusing, and phase adaptation. For the soft X-ray undulator, the setup is similar but has less segments.One prerequisite for achieving lasing is the tuning of the K-value of all undulator segments to a very high precision. This magnetic tuning of the undulators is done in the magnetic lab before the transport to the final position in the tunnels. When mounted in the tunnels, there is no possibility to measure the K-values directly by means of magnetic measurements. The undulator commissioning spectrometer, also known as the “K monochromator” or “K mono”, will make it possible to measure the spontaneous radiation of the undulator segments for K-tuning and other diagnostic measurements (e.g. pointing and phase matching).