  We present infrared spectra of brown dwarfs with spectral types from mid-L to
T. The 0.9-2.5 $\mu$m spectra of three dwarfs found by the Sloan Digital Sky
Survey contain absorption bands of both methane and carbon monoxide and bridge
the gap between late L and previously observed T dwarfs. These dwarfs form a
clear spectral sequence, with CH$_{4}$ absorption increasing as the CO
absorption decreases. Water vapor band strengths increase in parallel with the
methane bands and thus also link the L and T types. We suggest that objects
with detectable CO and CH$_{4}$ in the H and K bands should define the earliest
T subclasses. From observations of bright (K~$\leq$~13 mag) L dwarfs found by
2MASS, we find that the onset of detectable amounts of CH$_{4}$ occurs near
spectral type L5. For this spectral type methane is observable in the
3.3~$\mu$m $\nu$$_{3}$ band only, and not in the overtone and combination bands
at H and K.
