"Observers of democracy and democratization generally choose, implicitly or explicitly, among four main types of definitions: constitutional, substantive, procedural, and process-oriented. … A constitutional approach concentrates on laws a regime enacts concerning political activity. Thus we can look across history and recognize differences among oligarchies, monarchies, republics, and a number of other types by means of contrasting legal arrangements. Within democracies, furthermore, we can distinguish between constitutional monarchies, presidential systems, and parliament-centered arrangements, not to mention such variations as federal versus unitary structures. For large historical comparisons, constitutional criteria have many advantages, especially the relative visibility of constitutional forms."