Spectral sequence for B-type dwarfs. Stars in the left panel present numerous O II lines (some of which are mixed with C III features in objects earlier than B1), together with Si II/III/IV features that provide a powerful temperature discriminator. Stars in the right panel have simpler spectra. Objects later than B5 present some Fe II lines, but they tend to rotate fast and all weak features become very shallow and broad, as in the B8 V standard 18 Tau, a member of the Pleiades.