Logic of the astrometric solutions contributing to Gaia DR1. The top boxes show the number of sources input to the solutions, and the different priors used to constrain the solutions. The number of sources finally kept in Gaia DR1 is substantially smaller. The TGAS and the auxiliary quasar solutions work in the Hipparcos reference frame; the final primary and secondary solutions are aligned with ICRF. All sources are also treated by the secondary solution; in the end, a decision is made for each source whether to select the primary or secondary solution for Gaia DR1, or none. The results of the auxiliary quasar solution are only used for alignment, calibration, and validation purposes. The quasar results in Gaia DR1 come, with few exceptions, from the secondary solution.