By late in the last century it was realized that the spectra of stars (in particular, their patterns of absorption lines) had systematic features that could be classified into what came to be known as the Harvard Spectral Sequence. The sequence was denoted by a series of letters O, B, A, ... with the spectral characteristics listed in the following table.
For even finer gradation in the spectral sequence, each category in this classification can be subdivided into 10 subclasses using numbers from 0 to 9. Thus, for example, the classes O and B can be subdivided into the finer classes O0, O1, O2, O3, ... O9, B0, B1, B2, ... B9.
Here are some postscript files of example spectra for each of these spectral classes.