English: Simultaneously coloring the vertices and faces of a 1-planar graph may require at most six colors — for instance, in this triangular prism, no two of the 11 adjacent features of the same type (5 faces and 6 vertices) may take the same colors along an edge (shown here using an black color not counted), and no two of the 18 connection pairs of vertices and faces may take the same colors. Ringel conjectured in 1965 that six colors always suffice; this was proven in 1984 by Borodin. This coloring problem formed the inspiration for 1-planar graphs.