The Summer Sea is one of the major oceans of the known world. It is located to the south of both Westeros and Essos, separating those continents from the southern landmass of Sothoryos. It is separated from the Narrow Sea by the islands known as the Stepstones, and from the Jade Sea by the Straits of Qarth. Slaver's Bay and the Gulf of Grief are large inlets of the Summer Sea, and the Valyrian Peninsula extends out into it.
Little of mainland Sothoryos has been explored (only some of the northern shores), and the coasts rarely remain settled for long due to dangerous animals and diseases in the wilds of the local jungles. Three major settled regions, however, have developed on the different island groups in the Summer Sea off the coast of Sothoryos. Immediately north of Sothoryos are the Basilisk Isles, brutal pirate dens and slaver bases which are even worse than the Stepstones. West of mainland Sothoryos is Naath island, whose peaceful inhabitants are the targets of frequent slaver raids from Essos. Naath is directly south from Valyria, and located between Sothoryos to the east and the Summer Isles to the west.
The Summer Islands are located in the Summer Sea, west from the coast of Sothoryos and far to the southeast of Westeros. The Summer Islanders are a thriving civilization of sea-farers, whose merchant vessels can be found throughout the Summer Sea. The large archipelago is home to several port cities. The northernmost of the Summer Islands is directly south from the Narrow Sea, but the archipelago curves to the southeast, so the southernmost isle is much farther away from Westeros. There are no major islands in the Summer Sea located west of the Summer Isles (and thus no major islands or landmasses south of Westeros itself).
Several of the southern Free Cities are major ports located on the Summer Sea: Volantis is locate at the mouth of the Rhoyne River on the southern coast, and Lys is actually located on a group of off-shore islands surrounded by the warm waves of the Summer Sea. Tyrosh is located at the east end of the Stepstones, where the Summer Sea transitions into the Narrow Sea.
In Westeros, the southern coasts of Dorne and the Reach are on the Summer Sea. Oldtown, the second largest city in the Seven Kingdoms, is thus located on the Summer Sea, being situated in the southwest corner of the Reach (and therefore, the southwest corner of Westeros as a whole). The large wine-producing island known as the Arbor is located near Oldtown, separated from the mainland of the Reach and sitting in the Summer Sea. Planky Town, Dorne's main port settlement at the mouth of the Greenblood River at the east end of the peninsula, is also located on the Summer Sea.
As a ship rounds the southwest corner of Westeros at the Arbor and Oldtown, the Summer Sea turns into the Sunset Sea. Unlike the clear border formed between the Summer Sea and the Narrow Sea by the Stepstones, the Sunset and Summer Seas merge over vast stretches of open water, so far away from the mainland the end of one sea and beginning of another is somewhat vaguely defined. On a practical level this is not normally a cause for confusion, however, because there aren't any notable islands out in the middle of these oceans directly south or west of the Reach.
In the books
In the A Song of Ice and Fire novels, the Summer Sea is one of the largest bodies of water in the world, outstripped only by the Sunset Sea lying to the west of Westeros. Other than the west coast of Westeros, however, the Sunset Sea is devoid of any large island groups - explorers who have ventured west of both Westeros an the Summer Isles report finding only vast empty reaches of open ocean. The Shivering Sea on the north side of the Narrow Sea is also quite large, but it has comparatively few major cities in it. In contrast, many of the world's great civilizations - Westeros, the Free Cities, the Summer Isles, Valyria, Slaver's Bay, and Qarth - all have at least some ports on the Summer Sea.
The eastern end of the Summer Sea is separated from the Jade Sea farther to the east by an island known as Great Moraq - the largest island in the known world. The Summer Sea does connect to the Jade Sea, through the straits of Qarth at the north end of Great Moraq, and at the southern end the Cinnamon Straits between Great Moraq and Sothoryos. Due to trade winds and the direction of currents, trade ships need to move around Great Moraq in a clockwise direction: ships traveling west to east out of the Summer Sea need to pass through Qarth to the north, while ships making a return journey back into the Summer Sea need to go around the southern end of Great Moraq. Great Moraq is inhabited with some merchant cities but few details about it have been given in the novels, or even in the World of Ice and Fire sourcebook (2014). Great Moraq hasn't been mentioned yet in the TV continuity, in part due to using older maps.
The TV series only started depicting maps of the lands east of Westeros starting in Season 2, which were based on early drafts of world maps that George R.R. Martin created. As a result the only maps provided by HBO for the TV series so far depict the Jade Sea as extending north of the Summer Sea and the straits of Qarth. Martin, however, lated revised the world map and officially released it in The Lands of Ice and Fire collection, in which Qarth is actually located at the north shore of the Jade Sea, and the rest of it extends to the south. It is unclear if the TV continuity will be updated to take this into account.
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