What is the purpose of connecting two zener diodes back to back?

The back to back connected zener diodes can be used as an AC regulator producing what is jokingly called a "poor man's square wave generator". Using this arrangement we can clip the waveform between a positive value of +8.2V and a negative value of -8.2V for a 7.5V zener diode. If we wanted to clip an output waveform between different minimum and maximum values for example, +8V and -6V, use would simply use two differently rated zener diodes.

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