M _avg vs. H _avg for M31 cluster spectra, with different types identified. Model indices derived from SB99 models using Padova Z = 0.03 isochrones are also shown as a solid curve. Ages in Gyr are marked at selected areas along the curve. Median error bars are shown on the right for young and old clusters separately. The maximum errors on points in this plot are 0.45 for M _avg and 0.19 for H _avg.