A revolutionary wave, sometimes revolutionary decade, is a series of revolutions occurring in various locations within a particular timespan. In many cases, past revolutions and revolutionary waves have inspired current ones, or an initial revolution has inspired other concurrent "affiliate revolutions" with similar aims. The causes of revolutionary waves have become the subjects of study by historians and political philosophers, including Robert Roswell Palmer, Crane Brinton, Hannah Arendt, Eric Hoffer, and Jacques Godechot.