In electrophoresis, the introduction of a sample into the BGE creates two new zone boundaries, the front and rear boundary of the sample zone which is sandwiched between the BGE. During an electrophoresis run, these boundaries move and split into other new boundaries demarcating the zones of analytes. Besides the analyte zones migrating out of the original sample location, system zones may be also formed. Such zones are formed by the BGE components and differ from the BGE only in their concentrations. The front and rear boundaries of such system zones evolve during electromigration and may show sharp (S), dispersed (D), and/or hybrid character. This contribution brings the results of theoretical and experimental investigation of system properties of very simple BGEs formed by one weak acid and one weak or strong base provided that the acid is polyprotic. It is shown that system boundaries of the hybrid type occur even in these simple systems represented, e.g., by the very common phosphate buffer. Theory reveals that a phosphate buffer system (formed by phosphoric acid and a strong cation) may exhibit a very unusual complex shape of the electromigration dispersion velocity curve showing three turns. This leads to the formation of complex hybrid boundaries having D-S-D, S-D-S, or even S-D-S-D or D-S-D-S concentration profile types that have not been reported so far. The velocity diagram method allows theoretical analysis of such complicated profiles and the results are in good accordance with both computer simulation and experiments.