Example of the topology of a separatrix dome from one separatrix surface. a) 3D view of the topology consisting of a negative null (blue dot), its spine lines (thick purple lines) and separatrix surface field lines which arch down to the photosphere forming a dome (thin blue lines). The HCS null line (thick green line) and HCS curtain field lines (thin green lines) are also shown. b) Magnetic field two grid points above the photosphere (grey-scale contour map) with the intersection of magnetic skeleton at this height over-plotted, including the separatrix dome (blue line), spine (purple cross) and HCS curtains (green lines).