Game of Thrones, over the span of eight seasons, focused on only a few regions of Westeros, raising questions about the Seven Kingdoms and why they are called that when there are actually nine of them. Despite the show and its spinoff series House of the Dragon primarily exploring the Seven Kingdoms and regions of Essos, the origin of this nomenclature is never explained.

To understand the genesis of the Seven Kingdoms, one must go back to the time of Aegon's Conquest, which occurred around 300 years before the events of Game of Thrones and House of the Dragon. Aegon Targaryen, accompanied by his sister-wives Rhaenys and Visenya, arrived at Blackwater Rush in a Westeros divided into seven realms.

These separate realms eventually came under the rule of House Targaryen and were united as the Seven Kingdoms.

In Game of Thrones, viewers were introduced to some of these kingdoms, such as the North led by House Stark and King's Landing, while others like Highgarden, home of House Tyrell, were mentioned but not shown. Each kingdom had its distinct landscape and political practices. For instance, the Vale had a unique form of punishment where prisoners would be forced to fall through a floor gate to their deaths. Dorne featured a blend of desert and tropical regions and had a standard-looking prison.

Casterly Rock, home to House Lannister, and the Riverlands ruled by House Tully, made sporadic appearances in the show. The Iron Islands, known for their dark and stormy nature, contrasted with the dangerous seas surrounding them. However, the rest of Westeros remained largely unexplored in Game of Thrones, and the first season of House of the Dragon did not shed much light on these other regions either.

The Seven Kingdoms were initially called that when Aegon Targaryen was declared "Lord of the Seven Kingdoms" upon his coronation. However, this wasn't entirely accurate as Dorne fiercely resisted Aegon's conquest, and it was only 187 years later, through a peaceful marriage pact, that Dorne officially joined the realm.

This expansion turned the Seven Kingdoms into nine.

Aegon's Conquest also brought about changes in the realm. The Riverlands, previously independent, were divided into the Kingdom of Rivers & Isles, with House Tully ruling the Riverlands and House Greyjoy governing the Iron Islands. Additionally, Aegon established the Crownlands, a principality loyal solely to the crown and encompassing King's Landing and its surroundings.

Therefore, the official Kingdoms of Game of Thrones and House of the Dragon include the North, the Iron Islands, the Riverlands, the Crownlands, the Reach, the Stormlands, the Vale, the Westerlands, and Dorne.

However, following Robert's Rebellion and the War of the Five Kings, there were further changes to this status quo.

House of the Dragon has expanded its filming locations, which may allow for the exploration of new places within Westeros. Based on the Fire & Blood book that it adapts, future seasons of House of the Dragon could delve into Winterfell, the Reach, the Iron Islands, the Vale, and more during the Targaryen civil war known as the "Dance of the Dragons."

While the existing series never addressed the remaining Seven Kingdoms and their history, future Game of Thrones spinoffs have the potential to explore this part of the lore.

A series centered around Aegon's Conquest could provide insights into the origin of the Seven Kingdoms, though it may not fully answer the question about the nine realms. Alternatively, a brief history lesson from a character within the show could provide the explanation fans have been seeking and add to the world-building of the franchise.