We use recently calculated electron collisional data by Storey et al. (2005A&A...433..717S) for Fe XII (n=3 configurations) to build a model ion to compare to spectroscopic observations of the solar corona and laboratory measurements. We review all the line identifications of the brightest lines, by comparing both wavelengths and line intensities. The presence of blends in different plasma conditions is highlighted. A large number of new energy levels and line identifications are proposed. In particular, we identify the electric quadrupole ^4P_5/2_-^4^F_9/2_ transition with the bright coronal line observed at 592.6{AA}, previously unidentified. This line, when observed with the 1242.0, 1349.5 UV forbidden lines, is a good density diagnostic over a large range of electron densities. We also indicate the best spectral lines to be used for density diagnostics or instrument calibration. From various combinations of lines we obtain electron densities that are significantly different from those published in the previous literature. Some examples are provided. The agreement between theory and experimental data in terms of wavelengths, line intensities and level lifetimes is excellent.