'Arnor' is used in this title as a historical reference to the northern lands of the Dúnedain. The realm of Arnor itself had ceased to exist as a unified state in III 861, more than two thousand years before the War of the Ring, but nonetheless the old name was clearly still understood as referring to the homeland of the Northern Dúnedain.
These northern lands had seen strife and conflict during the time they were formally known as Arnor, and also for long afterwards, and so doubtless there would have been other Captains of the Dúnedain of Arnor over the millennia before Aragorn used the title for himself. Nonetheless, Aragorn's choice of this title in III 3019 to hide his true authority is the only recorded use of the term.
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