M9 (Figure 2) and persist into the L-dwarf range, becoming broader at L0 (Figure 3), and then undetectable by L5 (Figure 4). From T0 to T4.5 (Figures 5 and 6), order 64 becomes increasingly chopped up by new spectral features, some of which are quite sharp and deep. In section §3.3 we show that these features are caused by absorption by 2o.

Finally, there is order 65, which contains the second K I doublet and exhibits some of the largest changes with spectral type. The slightly weaker K I companion line at 1.17728 $\mu$m, only 3.3Å  from the longer wavelength member of the doublet is easily resolved in the M9 object (Figure 2), already blended from line broadening in the L0 (Figure 3), barely discernable at L5 (Figure 4), and completely washed out by line broadening and numerous molecular features at T4.5 (Figure 6). Order 65, being close to the short wavelength edge of the $J$-band where terrestrial water vapor absorption is expected, also contains many strong intrinsic transitions of hot H$_{2}$O, for example, the feature at 1.175 $\mu$m.

Al I, Fe I and Mn I; indicators of the M-L transition
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Figure 7 provides a more detailed view of 