Duchy (Crusader Kings 2)

A duchy is a realm of the third tier. Duchies are the second lowest ranked playable realm. Duchies are composed of 1 or more counties. The ruler of a duchy is titled Duke (male), Duchess (female), or an equivalent cultural title variant. Dukes can be independent rulers, but are more often vassals of powerful kings and emperors.

One of the counties in each duchy is the capital and is normally held by the duke of that duchy. If that county's capital is a city or temple then the realm is a republic or theocracy respectively. Kingdoms and empires  can also be republics and theocracies if the capital of the realm is a city or temple.

Cultural Variants
For most cultures, the ruler of a duchy is a duke (male) or duchess (female), but there are several exceptions: Note religous title variants 'trump' cultural ones. For example, a duke with Mongol culture and Tengri pagan religion will be known as a High Chief.
 * Doux/Duchess (Greek)
 * Emir/Emira (Arabic, Iranian)
 * Beylerbey/Beylerbayan (Turkish)
 * High Chief/High Chief (Mongol)
 * High Chief/Duchess (Pagan)
 * Jarl/Duchess (Norse Pagan)
 * Farbas/Farba (West African)
 * Grand Prince/Grand Princess (Russian)
 * Huecalpixqui/Huecalpixqui (Nahua)

Similarly, duchies themselves can also be known by other names: The titles of duchies and dukes in republics and theocracies are different still.
 * Emirate (Arabic, Iranian, West African)
 * Beylerbeylik (Turkish)
 * High Chiefdom (Mongol)
 * Grand Principality (Russian)