During the last three decades, there has been a growing interest in systems that combine elements of parliamentarism and presidentialism. Despite the fact that much attention has been directed towards the semi-presidential form of government in particular, it is evident that many aspects of regime forms remain unexplored. This book systematically categorises democratic political regimes with a separate head of state and government (including regimes with a monarch and prime minister, and president and PM) globally and over a long historical period 1850–2019. It analyses how regimes with a dual executive emerge and what trajectories they follow. It also explores the stability of these regimes across time and space. An important feature of this endeavour is to address actual powers of the head of state rather than constitutional provisions.