Spectral sequence for luminous B-type supergiants. Left panel shows the evolution of the Si III/Si IV ratio, the growth of the O II spectrum to a maximum at B1, and its subsequent decrease, and the rise of the N II spectrum to a maximum at B2. Right panel illustrates the smooth growth of the Si II doublet and Mg II 4881 Å with respect to the neighbouring He I lines. Si III, O II and N II are essentially gone by B6, but the Fe II spectrum is already growing. We note that some of these spectra, unlike those of the main sequence stars, contain deep diffuse interstellar bands, which are more prominent in the spectrum of HD 183143, where the broad band at ∼4430 Å is the strongest feature. Most of Garrison’s anchor standards for the MK system (cf. Table 1) are included in this sequence.