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The Bronx is referred to, both legally and colloquially, with a definite article, as the Bronx. (The County of Bronx, unlike the coextensive Borough of the Bronx, does not place the immediately before Bronx in formal references, nor does the United States Postal Service in its database of Bronx addresses.) The name for this region, apparently after the Bronx River, first appeared in the Annexed District of the Bronx created in 1874 out of part of Westchester County and was continued in the Borough of the Bronx, which included a larger annexation from Westchester County in 1898. The use of the definite article is attributed to the style of referring to rivers. Another explanation for the use of the definite article in the borough's name is that the original form of the name was a possessive or collective one referring to the family, as in visiting The Broncks, The Bronck's or The Broncks'.

What county was the Bronx split off from?
Westchester