Optimization of linking lengths for the friends-of-friends (FOF) group finding algorithm. The top row shows results for TNG; the bottom row shows results for EAGLE. Each column displays a redshift snapshot (as labeled for each panel). Separate sky (l_sky) and redshift (l_z) lengths are used as the maximum threshold for linking galaxies together in the FOF procedure. The X-axis displays varying l_sky values, which are measured in physical distances, with varying l_z values shown by different colored lines (see legends), which are measured in velocity space. Optimization is performed with respect to the total accuracy of central - satellite classification (shown on the Y-axis). The optimal values are presented in each panel.