Baseline representation for a two-telescope interferometer according to the GRAVITY concept. The baselines are defined by their limit points. The WAB is defined by the two pivot points (which defines the center of the telescopes reference frames). The NAB is defined by the geometrical average of the position of the four metrology photodiodes situated on the spider arms. The coordinate of the NAB limit points are (x, y, z) = (0, 0, 10 m). The two imaging baselines IMB₁ and IMB₂ (one for each beam combiner) have limit points that correspond to the pupil position of their respective beam combiners, weighted laterally by the telescope aperture, as well as the electric-field acceptance of the single-mode fibers. Internal pupil tracking inside the GRAVITY instrument will track the spider arms to place the IMB limit points on the NAB limit point. Expected lateral position will be |x,y| = 0 ± 4 cm. Expected longitudinal position will be z = 0 ± 10 km.