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The gaseous component of the the Milky Way and nearby galaxies have phase structure (i.e. spatial locations with different densities and/or temperatures). Examples are the disk/halo interface (Galactic coronae) and the cold, warm, and hot phases of the interstellar medium. From photoionization models, it is not usually possible to generate absorption that is simultaneously consistent with all observed chemical transitions for a given system. For example, in single cloud Mg II systems, Figure 6 (with N(HI) = 1016 cm -2) shows that if Fe II is detected at a similar column density to Mg II, the ionization parameter must be small, and Wr(C IV) cannot be large. Many systems have C IV absorption which exceeds this limit and requires a higher ionization (lower density) phase; generally, this phase must have structure over a large velocity range (a large ``effective'' Doppler parameter). The z = 0.93 system toward the quasar PG 1206+459 is another case that requires multiphase structure. The observed C IV profile in Figure 4 is much too strong for this absorption to arise in the same clouds that produce the Mg II, even if their ionization parameters are pushed to the largest values consistent with the data.