Okay, here goes. Hold on tight, though, this gets a little complicated.
Immediately before the great consolidation of 1898 NYC consisted of Manhattan Island and what we now call the Bronx. Manhattan Island was always NYC, since colonial days. The Bronx was added to NYC by taking two bites out of lower Westchester County: the area west of the Bronx River joined NYC in 1874, and the area east of the river joined in 1895. The Bronx did not get its present name until the borough system went into effect in 1898. Before that, that piece of NYC was called the “Annexed District” or the “North Side.”
(It is worth noting that when those two pieces of the Bronx / Annexed District / North Side were added to NYC, they were also added to New York County. When the boroughs were created in 1898 *they stayed part of New York County! *It wasn’t until the 19-teens that Bronx County was created. Bronx residents were tired of heading all the way to Downtown Manhattan to serve jury duty, etc.)
Brooklyn was the only neighboring municipality that could compete with NYC in terms of population and prestige in 1898. It was the third or fourth largest city in the country at the time. It itself had grown in stages through the 19th century, annexing the nearby towns of Kings County. It annexed its last town around 1896 (?). By attaching that last town, the City of Brooklyn became coterminus with Kings County. When the borough system came into effect, the City of Brooklyn became the Borough of Brooklyn.
In 1898 Staten Island consisted of a patchwork of towns and villages. All the municipalities on SI, taken together, comprised Richmond County. When the borough system went into effect, the municipalities on Staten Island were joined to became the Borough of Richmond. The borough would be renamed “Staten Island” in the 1970s; but the county retained the name “Richmond” – as it does to this day.
Queens, as the discussion in the previous thread indicates, was the the messiest area. In the late 19th century Queens County stretched from the East River to the Suffolk County line. The county contained Long Island City (yes, a fully chartered city) and a patchwork of towns and villages stretching to the east. The consolidation of 1898 created Queens Borough from the westernmost third (roughly) of the county. This nutty arrangement – having part of Queens County in NYC and part of it outside – only lasted a year. In 1899 Queens County was reduced to the area of Queens Borough, and Nassau County was created to contain the territory between the Queens County and Suffolk County lines.
Got it? Good!
BTW, discussions of the consolidation of 1898 will often ignore or diminish the significance of the three “minor” boroughs – the Bronx, Staten Island, and Queens – in the process. This is not for reasons of outer-borough prejudice or elitism as is often presumed. If you look at population numbers of those three districts in the late 19th century you’ll see that they were severely underpopulated and undeveloped, at least compared to Manhattan and Brooklyn. Their lack of influence was just a reflection of their small populations.