Schematic image of the structure of the Weyl semimetal in momentum space. Two diabolical points are shown in red, within the bulk three-dimensional Brillouin zone. The excitations near each diabolical point behave like Weyl fermions. Each point is a source or sink of the flux (i.e., a monopole in momentum space) of the U(1) Berry connection, defined from the Bloch wave functions, as indicated by the blue arrows. The grey plane indicates the surface Brillouin zone, which is a projection of the bulk one. Wan et al. show that an odd number of surface Fermi arcs terminate at the projection of each diabolical point, as drawn here in yellow. In the iridium pyrochlores studied in the paper, a nonschematic picture would be significantly more complex, as there are 24 diabolical points rather than 2.