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We present high-resolution IUE spectra of the emission cores of the Mg II resonance doublet at 2800 in a selection of all F, G and K main sequence stars available in the IUE archive. We discuss the qualitative line profile groupings. We also derive chromospheric radiative losses in the h and k lines, and discuss these loss rates as functions of effective temperature. Chromospheric velocity fields and indicators of circumstellar envelopes are discussed in terms of profile asymmetries and other diagnostics. Finally, we discuss the relationship between the h and k line widths and absolute magnitude.