A government or system ruled by two leaders or authorities with equal power.
From Greek 'bi-' meaning two and 'arkhia' meaning rule or government. Parallel to 'monarchy' and 'anarchy,' created in the 17th century.
Ancient Sparta had two kings as a biarchy—one unexpected benefit was that they checked each other's power, making absolute tyranny mathematically harder (though they still managed oppression masterfully).
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