The standard parcel of land in New York City (well, Manhattan at least) after the Commissioners' Plan of 1811, was 25' x 100'.  This size - a 25' wide parcel - was too large for 'standard' middle-class townhouses, yet too small to be of much use if cut in half (two, 12.5' wide buildings).  As a result, most 19th century developers tried to buy at least two parcels side by side, and aim for 16-18' wide buildings. Townhouses that were sized between 16’ and 18’ maximized profit by employing fairly standard wooden beams (an 18' beam or less was readily available, but once you wanted 18’-25' long beams, costs skyrocketed).

If a developer bought two lots, and thus had 50' of street-side frontage, he could then easily and efficiently build 3 units, side by side, with shared party walls (another cost saving) of about 17' wide, each.  Part of the strategy to build many of these 3-in-a-row townhouses in as cost-effective a way possible required that the workers and tradesmen be able to quickly get from each of the 3 townhouses to the one/s adjacent.  As a result, many brownstones or townhouses have hidden (and subsequently filled in) passageways that went from neighboring building to neighboring building.

During renovations, you can sometimes see this hidden passage as a filled-in (often arched) space, typically in the front hallway, at the top of the stoop.  The photos included here show these hidden passageways that were bricked up once most of the construction was over.

Drinking in NYC

Really? Riverdale and East Harlem both report 0% heavy drinking?

Learn more, at Vital City.