A more thorough explanation:

A duke is a ruler of a duchy or a sovereign prince. In Great Britain, it is the first order of nobility below the royal family.

Before the Norman conquest, kings did not give the title of duke to anyone. However, in the eleventh year of Edward III's reign, he created his son, Edward the black prince, duke of Cornwall. Later, many members of the royal family were also given the title. The order of duke became extinct during the reign of Queen Elizabeth in 1572 but was revived about fifty years later by her successor, George Villiers, duke of Buckingham.

For example, Prince William, the Duke of Cambridge, is a member of the British royal family and holds the title of duke. He is not a sovereign prince but is still considered a high-ranking member of the nobility.

Another example is the Duke of Wellington, who was a British military commander and politician. He was given the title of duke as a reward for his military victories.

These examples illustrate how the title of duke is given to individuals who have achieved great success or have a high social status.