The term Britannia is, in Ultima Online, often used incorrectly as a synonym with Sosaria. Properly, however, it refers to the original UO map, the one with the cities of Trinsic, Britain, and Yew; the Kingdom created by Lord British's consolidations of the realms found in the single-player game Ultima I.
Britannia and the Lost Lands are located on a planet that is sometimes called Sosaria. Whether or not the other lands/facets from modern UO are located on that planet is not clear and is subject to dispute. Some might be (Tokuno), some definitely are not (Malas and Ter Mur).
In EA/OSI's version of UO, the land once called Britannia has been known as the facet of Felucca since the release of the Renaissance expansion and the introduction of the facet of Trammel.
Britannia in Oceania Classic
Oceania Classic features Britannia before the Felucca/Trammel split. When we talk about the land of Britannia we are referring to what later became Felucca in EA/OSI's version of UO.
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