Transmission-line structures are commonly used to support guided wave propagation. To that end, ordinary transmission lines utilize two parallel electric wires of very high conductivity. Very recently, another option has emerged, the so-called magnetic transmission line (MGTL), which utilizes two parallel magnetic wires of very high permeability. The theory of magnetic transmission lines is not completely settled; adjustments are still needed, and its own vocabulary requires some tuning as well. In this paper new grounds are established for MGTL theory, encompassing two-wire and multi-wire systems. For that purpose the new concepts of magnetic charge, magnetic current and magnetic voltage are introduced and discussed. These conceptual tools are made use not only to describe field equations in the transverse plane but also field-wave equations in the propagation direction. With these new tools, the duality between electric and magnetic transmission line analysis becomes self-evident.