A unified, rigorous and efficient hybrid-mode solution to the general planar 3-layer transmission-line problem with shielded, covered and open cross-sectional geometry is achieved by the spectral-domain method. Stress is put on the minimisation of the computational expense required to obtain the frequency-dependent design data of planar line structures with realistic geometrical dimensions. This implies the modal propagation constants, uniformly defined characteristic impedances and any field quantities. On the basis of user-oriented considerations a computer program has been developed which is applicable to fundamental, and higher, even and odd modes on single and coupled coplanar strips and slots. This computational approach includes most of the cases which today are of technical interest, e.g. stripline and microstrip, microstrip with dielectric overlay, suspended substrate strips and slots, slotline, coplanar line and grounded or isolated fin line. It combines analytical simplicity, rigorousness and high efficiency with the possibility of performing studies on numerical accuracy and convergence.