Galaxy merger tree of Aquarius halo A-4, showing only those objects that contribute at least 0.1% to the total stellar mass of the accreted spheroid (in this example this corresponds to galaxies with a minimum stellar mass of 4.5 × 10⁶M_⊙). Only if a building block of a building block itself has a stellar mass above this threshold is it also shown. Building block 12 is the largest progenitor building block, contributing 45% of the accreted spheroids’ stellar mass, followed by BBs 5, 10, and 6, which contribute respectively 31%, 10%, and 4.9%.