There are 5 principal political systems: theocratic, monarchal, single-party, military, and democratic.
Once ubiquitous, ruling under religious auspices is now confined to Islamic countries, and of course Vatican City, the Roman Catholic papal city-state.
Monarchies were the norm until the rise of the middle class, beginning in the 18th century, whereupon democratization took hold. Yet 44 countries, including 16 realms under Britain’s Queen Elizabeth II, still have a monarch as head of state. Only 2 monarchies went out in the 21st century. The last Samoan king died, leaving a parliamentary democracy. In Nepal it took a combination of a communist rebellion, a populist uprising, and a murderous prince, who killed 10 royal family members, to bring down the kingdom.