Dragonstone is the ancestral seat of House Targaryen. Prior to Robert's Rebellion, the island served as the seat of the heir apparent, known as The Prince of Dragonstone. Dragonstone is given to Stannis Baratheon by his brother, King Robert after he won the Iron Throne. Since Robert did not grant his brothers the title "Prince" (reserving that title for his own sons and the ruler of Dorne), Stannis was styled the Lord of Dragonstone instead.
Lords of Dragonstone
In the books
"Prince of Dragonstone" is the title of the heir apparent of the Seven Kingdoms. Dragonstone became the seat of the heir once the Targaryens established King's Landing as the capital of the Seven Kingdoms. After the War of the Usurper, the title was not officially used. Daenerys Targaryen was styled "Princess of Dragonstone" by the handful of Targaryen supporters left, but after her brother's death claimed the Iron Throne instead and ignored the title. Stannis Baratheon became "Lord of Dragonstone".
In the time between the Doom of Valyria and Aegon's Conquest, the Targaryen rulers of the island used the title "Lord of Dragonstone".
The use of a specific princely title for the heir to the throne parallels real-world monarchies: the heir to to the King of the United Kingdom (and before that, the King of England), is granted the title "Prince of Wales", while the heir to the throne of Spain is traditionally the "Prince of Asturias".
|Lord:||King Stannis Baratheon||Heir:||Princess Shireen Baratheon|
|Seat:||Dragonstone||Lands:||Blackwater Bay and the Stormlands|
|Title(s):||Lord of Dragonstone|
|Current members:||Queen Selyse Florent|
|Household:||Maester Cressen · Lady Melisandre · Ser Davos Seaworth · Ser Imry Florent · Matthos Seaworth · Bert|