Why is the peak inverse voltage in a full wave rectifier 2VMAX?

Assuming that the rectifier will be followed by a filter capacitor, the p.i.v. should be at least twice the peak of the applied a.c.

(The capacitor will charge to the peak of the applied a.c. On the next half cycle of the input, the peak of that cycle will be of the opposite sign to that of the stored voltage on the capacitor, so the two add - giving twice the peak.)