The new classification systems just outlined have two strong similarities to the MK System: the boxes that comprise them are defined in each case by the spectrum of a single star; and the occupants of each box resemble each other closely in their spectral appearance. There are, however, certain categories of stellar spectra that could require a different approach; these categories could be labeled as stellar spectra which contain an outstanding feature (line, blend, pattern) that discriminates them from the ordinary inhabitants of the boxes of the MK System.
In such cases, it could be of considerable importance to make the first stage in classification a simple segregation of all spectra containing such a feature into a single ``basket.'' The inhabitants of such a basket could represent wide varieties of stellar spectra, but would have the single property - the outsanding feature - in common.