Most problems in electromagnetics require a three-dimensional grid. As we now have structures that describe TE and TM-field distributions, intuitively it seems possible to define a combination of shunt and series nodes providing a full description of the electromagnetic field. Such attempts have been made, but because of the complexity of the resulting structures they proved to be not very useful. Using the analogy that was presented above leads to calculation of the different field components at physically separated points. This causes difficulties in providing simple and efficient boundary definitions. A solution to these problems was provided by Johns in 1987, when he proposed the structure known as the symmetrical condensed node (SCN), presented in the figure on the right. It consists of 12 ports because two field polarisations are to be assigned to each of the 6 sides of a mesh cell.