Spectral sequence of iPTF15dtg. In gray, we show the unbinned spectra, in black the same spectra after smoothing. The spectra are normalized by their median and shifted vertically for clarity. The spectra are de-redshifted but they are not corrected for extinction. For each spectrum we report its phase (in days since explosion). The main telluric feature is indicated by a gray area. Line identifications are reported for some of the main features.