Multi-line techniques for inference of stellar magnetic fields

Spectropolarimetric studies of stellar magnetic fields usually deal with extremely weak line polarisation signatures. The amplitudes of Stokes signals, even in magnetically sensitive spectral lines, are often well below the noise level realistically achievable with the current instrumentation. Consequently, a detection of these polarisation signatures and their meaningful analysis is impossible without combining information from multiple spectral lines. Here I review basic theoretical foundations of the multi-line spectropolarimetric diagnostic methods employed in stellar magnetometry, give examples of their application, and discuss recent efforts to interpret mean polarisation profiles with the help of detailed radiative transfer calculations.