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[en] We analyze observations from the Interface Region Imaging Spectrograph of the Mg ii k line, the Mg ii UV subordinate lines, and the O i 135.6 line to better understand the solar plage chromosphere. We also make comparisons with observations from the Swedish 1-m Solar Telescope of the Hα line, the Ca ii 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the coronal line. To understand the observed Mg ii profiles, we compare these observations to the results of numerical experiments. The single-peaked or flat-topped Mg ii k profiles found in plage imply a transition region at a high column mass and a hot and dense chromosphere of about 6500 K. This scenario is supported by the observed large-scale correlation between moss brightness and filled-in profiles with very little or absent self-reversal. The large wing width found in plage also implies a hot and dense chromosphere with a steep chromospheric temperature rise. The absence of emission in the Mg ii subordinate lines constrain the chromospheric temperature and the height of the temperature rise while the width of the O i 135.6 line sets a limit to the non-thermal velocities to around 7 km s−1.