The evolution of the Si(1 1 1) surface after submonolayer deposition of Ga has been observed in situ by low-energy electron microscopy and scanning tunnelling microscopy. A phase separation of Ga-terminated -R 30° reconstructed areas and bare Si(1 1 1)-7 × 7 regions leads to the formation of a two-dimensional nanopattern. The shape of this pattern can be controlled by the choice of the surface miscut direction, which is explained in terms of the anisotropy of the domain boundary line energy and a high kink-formation energy. A general scheme for the nanopattern formation, based on intrinsic properties of the Si(1 1 1) surface, is presented. Experiments performed with In instead of Ga support this scheme.