Only one electrical mode was permitted in the previous circuits and components. With modern RF components and high-speed digital circuits, however, two or even three modes occur. The different modes are distinguished by the electrical field images and possibly different propagation characteristics. This chapter 5 first explains which electromagnetic modes are involved. Furthermore, new "scattering parameters" are introduced for the most important two- and three-mode systems. Based on these mixed-mode parameters (M-parameters for short), the so-called symmetry transformers as well as new mode blockers and mode converters can be introduced in a simple manner. The application of these M-parameters is then discussed. It is shown how differential components such as antennas and high-speed ICs are matched. High-speed transceivers for digital signals are introduced. The use of ECL circuits in RF circuits is discussed. This is followed by the presentation of a new circuit synthesis technique for purely symmetrical networks. Based on these results, RF coupler circuits, which are used as pure four-port networks, can be precisely derived.