The Bronx street grid is irregular. Like the northernmost part of upper Manhattan, the West Bronx's hilly terrain leaves a relatively free-style street grid. Much of the West Bronx's street numbering carries over from upper Manhattan, but does not match it exactly; East 132nd Street is the lowest numbered street in the Bronx. This dates from the mid-19th century when the southwestern area of Westchester County west of the Bronx River, was incorporated into New York City and known as the Northside.