Integrated optics beam combiner: the actual interference happens in an etched silica-on-silicon integrated optics chip. The top panel shows a photograph of the GRAVITY integrated optics, the lower panel the design of the circuit. The integrated optics contain all functions (dark blue: waveguides, red and gray: beam splitter, light blue: phase shifter, green: X coupler) for a pairwise combination of the four telescopes and fringe sampling. The light from the four telescopes T1 ... T4 is fed by single-mode fibers from the left, the output on the right is the interference of the six telescope combinations for a relative phase shift of 0°, 90°, 180°, and 270°, respectively.