Revolution

§ A "revolution" occurs when you change your civilization's Civics or Religion.
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Effects of a Revolution

§ During a revolution your civilization descends into one or more turns of "anarchy." A civilization in anarchy produces no food, commerce, or production! Any progress towards completing buildings, units, or projects is stopped until the revolution is over.
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Duration of a Revolution

§ The duration of a revolt depends upon the number of civics that you change: one or two civics will usually result in one turn of anarchy, while three or more civics may cause two or more turns of anarchy. The civics screen tells you how many turns of anarchy you may look forward to after you choose which civics to change. If the length of time is too long, either make fewer changes or cancel them altogether until you can spare the time. Religions always have one turn of revolution.
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Immunity to Revolution

§ Civilizations with Leaders possessing the "Spiritual" trait do not suffer the effects of revolution or anarchy at all.
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Delay Period

§ After a revolution has taken place, there is a short period of delay before another revolution can be carried out. When the "Revolution" button appears once again on the civics screen, your citizens have recovered enough from the previous anarchy to carry out another revolution.