CoSy generates both purely Doppler-free and purely Doppler-broadened absorption saturation spectra, which can be fed directly to appropriate stabilization electronics. CoSy can conveniently be used for top-of-fringe stabilization with a modulation technique (bandwidth 50 kHz .. 1 MHz). The CoSy module helps the customer save time and space in his experiment. The absorption cell of the Cosy is filled with Rb, Cs and K and serves as a frequency reference for Rb, Cs and K frequencies from a single device.