object (Figure 2) with the M6 source (Figure 1) we see that the Al I lines at 1.3127 and 1.3154 $\mu$m in order 58 are somewhat weaker at M9 and then suddenly they are no longer present at L0 (Figure 3), or in any later spectral types (Figures 4-6). This is an important observation that relates to the M-L transition and we will discuss the Al I lines in the next section. Throughout order 58 there are other weaker spectral features, the so-called fine-scale spectral structure. This spectral structure becomes more pronounced at M9 (Figure 2), seems broader in the L0 and L5 objects (Figures 3 and 4), weakens at T0 or more accurately from L6 to T2 (Figure 5) and then completely changes character by T4.5 (Figure 6). The most likely interpretation of this spectral sequence is that it represents changes in the physical structure of these cool atmospheres (temperature, pressure, chemistry). In subsequent sections we compare opacity data for different molecular species to identify the primary absorbers at each spectral class.

In order 59 the sharp Mn I line at 1.2903 $\mu$m seen at M6 and M9 (Figures 1 and 2) broadens and disappears after L0 (Figure 3). 