This paper presents an atlas containing 59 low-resolution supernova
spectra which correspond to 27 type Ia supernovae. These 59 spectra spectra which correspond to 27 type Ia supernovae. These 59 spectra refer to different ages along the postmaximum type Ia evolution, covering from the photospheric to the old (~>300 days) nebular phases. These supernovae were observed as a part of the long-term observational program Supernova Monitoring Project, carried out at the Observatorio del Roque de los Muchachos (La Palma, Spain) from 1990 July to 1994 July. Most of these observations were taken with the Faint Object Spectrograph (FOS) and a few spectra were obtained by means the ISIS spectrograph. Three or more spectra were obtained for ten of these supernovae. Concerning the homogeneity among the type Ia supernova subclass, our sample points to a "first-order" overall homogeneous spectroscopic behavior. However, a finer analysis - comparing spectra at similar evolutionary phase-allows differences to be found in relation to strengths, wavelength positions, and evolution with time of the more significant spectral type Ia supernova features. Furthermore, some objects that exhibit clear signs of spectroscopic peculiarities (SNe 1991F, 1991bg, and 1991bj) are also included in our sample. The parent galaxies of these supernovae present redshifts ranging from z = 0.003 to z = 0.07. Four of these (SNe 1991S, 1991ad, 1992R, and 1992ac) were observed in galaxies with z~>0.05. Only six events in our sample were detected in an early-type (i.e., elliptical or lenticular) galaxy, whereas multiple supernova events were detected in five of the parent galaxies in our supernova sample.