Cross-section of the grid structure used for the binary black hole simulation, in the z = 0 plane. The two black holes are centred at (0, ± 5, 0). Two pairs of spherical grids (black and cyan, and blue and magenta) are used to capture the excision boundaries of the black holes, a Cartesian grid (green) surrounds them, and a third pair of spherical grids (blue and red) is used to capture the outer spatial boundary at 50M. This uses 4 195 765 grid points in total and 500 516 interpolation points.