Example of structure in chemical abundance space. For the system of merging galaxies shown in Figure 5, we plot the chemical abundances of stars (only showing low-mass stars that will still be alive at present day), colored by whether the star formed in Progenitor 1 (blue), Progenitor 2 (purple), Progenitor 3 (green), or later in the fully merged galaxy (orange). The data are binned on expected observational uncertainty for stars in the Milky Way halo today (the gray-white grid). The size of the circle in each bin is proportional to the log of the number of stars in that bin. In this simple example, the location of stars in multidimensional chemical abundance space clearly corresponds to which halo the stars were born in.