Montage of spectra of selected O supergiant, B hypergiant, WNVLh, and LBV stars to illustrate the evolution in strengths of the Si IV, Si III, and Mg II lines as a function of decreasing stellar temperature. In order to render these transitions visible, the He I2.058 μm and Brγ profiles of certain stars overlap; these are reproduced in Figs. 6 and 7. Note the appearance of pronounced emission in Fe II in the coolest star, the LBV qF362. See Sect. 3.1 for further details.