This contribution provides a comprehensive mechanistic picture of the gold nanoparticle (AuNP) synthesis by citrate reduction of HAuCl4 - also known as Turkevich synthesis - by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of the synthesis. Herein it is shown that this mechanism is actually a fortunate coincidence created by a favorable interplay of different chemical and physicochemical processes which initiate but also terminate the formation of very few seed particles and prevent the formation of new particles at later stages of the reaction. Since no new particles are formed after the seed particle formation, their number defines the final total number of AuNP. Therefore, the number of seed particles determines the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus final size.