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In law and government, de jure ( day JOOR-ee, dee -; Latin: de iure [de: 'ju:r?], "by law") describes practices that are legally recognised, regardless whether the practice exists in reality. In contrast, ("in fact") describes situations that exist in reality, even if not legally recognised. The terms are often used to contrast different scenarios: for a colloquial example, "I know that, de jure, this is supposed to be a parking lot, but now that the flood has left four feet of water here, it's a de facto swimming pool". To further explain this, even if the signs around the flooded parking lot say "Parking Lot" (the "law" determining it as a parking lot), it is "in fact" a swimming pool (due to the current practical circumstances of it being flooded).
It is possible to have multiple simultaneous conflicting (de jure) legalities, possibly none of which is in force (de facto). After seizing power in 1526, Ahmad ibn Ibrahim al-Ghazi made his brother, Umar Din, the lawful (de jure) Sultan of Adal. Ahmad, however, was in practice (de facto) the actual Sultan, and his brother was a figurehead. Between 1805 and 1914, the ruling dynasty of Egypt ruled as de jure viceroys of the Ottoman Empire, but acted as de facto independent rulers who maintained a polite fiction of Ottoman suzerainty. However, from about 1882, the rulers had only de jure rule over Egypt, as it had by then become a British puppet state. Thus, Egypt was by Ottoman law de jure a province of the Ottoman Empire, but de facto was part of the British Empire.
In U.S. law, particularly after Brown v. Board of Education (1954), the difference between de facto segregation (segregation that existed because of the voluntary associations and neighborhoods) and de jure segregation (segregation that existed because of local laws that mandated the segregation) became important distinctions for court-mandated remedial purposes.
In a hypothetical situation, a king or emperor could be the de jure head of state. However, if they are unfit to lead the country, the Prime Minister or chancellor would assumedly become the practical, or de facto leader, with the King remaining the de jure leader.