Building blocks of global coronal topology. This example is Carrington rotation 2074 (August 2008, solar minimum cycle 23/24). The Sun is shaded with the radial component of the magnetic field at the photosphere. Red and blue dots represent positive and negative null points. Separators are thick yellow lines. Separatrix surfaces are traced from the nulls, as well as from the null-line at the base of the heliospheric current sheet (HCS) at the upper boundary of the model (green line) and from bald patches at the lower boundary of the model. The HCS curtain is represented by the bright green field lines, bald patch separatrix surfaces (if present) are represented by the olive green field lines, separatrix surfaces from positive/negative nulls are represented by pink/blue field lines. Spine lines from positive/negative nulls are orange/purple.