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We report parallax measurements for 70 ultracool dwarfs (UCDs). Using bothliterature values and our sample, we report new polynomial relations betweenspectral type and M$_{JHK}$. Including resolved L/T transition binaries in therelations, we find no reason to differentiate between a "bright" (unresolvedbinary) and "faint" (single source) sample across the L/T boundary. Isolatingearly T dwarfs, we find that the brightening of T0-T4 sources is prominent inM$_{J}$ where there is a [1.2 - 1.4] magnitude difference. A similar yetdampened brightening of [0.3 - 0.5] magnitude happens at M$_{H}$ and a plateauor dimming of [-0.2 - -0.3] magnitude is seen in M$_{K}$. Comparing withevolutionary models that vary gravity, metallicity, and cloud thickness we findthat a near constant temperature of 1200 $\pm$100 K along a narrow spectralsubtype of T0-T4 is required to account for the brightening and color magnitudediagram of the L-dwarf/T-dwarf transition. Furthermore, there is a significantpopulation of both L and T dwarfs which are red or potentially "ultra-cloudy"compared to the models, many of which are known to be young indicating acorrelation between enhanced photospheric dust and youth. For the lowsurface-gravity or young companion L dwarfs we find that 8 out of 10 are atleast [0.2-1.0] magnitude underluminous in M$_{JH}$ and/or M$_{K}$ compared toequivalent spectral type objects. We speculate that this is a consequence ofincreased dust opacity and conclude that low-surface gravity L dwarfs require acompletely new spectral-type/absolute magnitude polynomial for analysis.

The Brown Dwarf Kinematics Project (BDKP). III. Parallaxes for 70 Ultracool Dwarfs