  We present an atlas composed of more than 1500 spectra of late-type stars
(spectral types from G to M) observed simultaneously in the optical and calcium
triplet spectral ranges. These spectra were obtained as part of a survey to
search for cool supergiants in the Magellanic Clouds and were taken over four
epochs. We provide the spectral and luminosity classification for each spectrum
(71% are supergiants, 13% are giants or luminous giants, 4% are carbon or S
stars, and the remaining 12% are foreground stars of lesser luminosities). We
also provide a detailed guide for the spectral classification of luminous
late-type stars, the result of the extensive classification work done for the
atlas. Although this guide is based on classical criteria, we have put them
together and re-elaborated them for modern CCD-spectra as these criteria were
scattered among many different works and mainly conceived for use with
photographic plate spectra. The result is a systematic, well-tested process for
identifying and classifying luminous late-type stars, illustrated with CCD
spectra of standard stars and the classifications of our own catalogue.
