The remarkable homogeneity of SNe la is the basis for their cosmological use. To a first approximation, their B-band light curves form a one-parameter family, so that the B maximum luminosity can be inferred from the light curve shape. Another possibility for luminosity calibration are spectral parameters that correlate with luminosity. The "luminosity indicator" which has received perhaps the largest attention is R(Si II) , the ratio of the strengths of the Si II λ5912 and λ6355 lines. Using radiative transfer models, we explain the R(Si II) behaviour as an ionization effect: Si II λ5972 is stronger in dimmer objects because of a much larger Si Il/Si III ratio, and Si II λ6355 shows no trend because of saturation. This explains the rise in R(Si II), but also leads to the conclusion that the strength of Si II λ5912 itself is a more direct spectroscopic luminosity indicator However, even the most direct spectroscopic luminosity indicators are likely to be affected by the intrinsic variability of SN la spectra. The resulting uncertainties may be minimised if several indicators  are used to obtain an average result and an error estimate. The accuracy of such a method remains to be investigated.