Calibration of the BCD (λ₁, D^*) plane in terms of the MK spectral type. The vertical dashed lines show the limits of the MK spectral type groups indicated on the axis of abscissas. The horizontal dashed lines represent the limits of the MK luminosity classes, which are indicated in the last vertical curvilinear quadrilaterals. Our sample of Be stars has luminosity classes in the V-II range, whilethe Bn stars show luminosity classes V or IV.