Layout of GRAVITY for MATISSE in the VLTI laboratory. The infrared light beams reaching the laboratory are first reduced from 80 mm to 18 mm by the beam compressors, then sent by the switchyard towards the MATISSE FOs, and finally split by the FOs between the near-infrared reflected to GRAVITY (green) and the mid-infrared transmitted to MATISSE (red). The light split between MATISSE and GRAVITY is implemented with a dichroic in transmission and one mirror reflection towards MATISSE, and the same dichroic in reflection and three mirror reflections towards GRAVITY. MARCEL is a coherent calibration source that can be used to co-phase MATISSE and GRAVITY.