The local blogosphere has been discussing the boundaries of Roxbury, Dorchester, and Mattapan. One popular blogger, known as the Whalehead King, hosts a popular and positive site talking about the Dorchester and Mattapan communities, and last week he wrote:

“The Roxbury-Dorchester border - I was reading an article about Open Air Boston describing how they brought wireless service to ‘the Grove Hall neighborhood of Dorchester.’ I remember when the service started and it was likewise described as being in Dorchester in the Globe.

The funny thing is, I was in Grove Hall this morning headed down Blue Hill Avenue to the Stop & Shop located in Grove Hall’s Mecca… headed from Franklin Park, the typical blue oval sign that announces Boston neighborhoods reads, ‘Welcome to Grove Hall/ Roxbury/ City of Boston/ Mayor Thomas M. Menino.’

“To my mind Dorchester has always ended at Columbia Road. I know it’s not a perfect or official boundary but it works well enough in my mind. I travel the length of Dudley Street everyday, and when I cross Columbia Road in Upham’s Corner, from Stoughton Street onto Dudley, my head knows I am still in the Dot, but my heart tells me I am entering Roxbury territory. In the opposite direction on Dudley Street, after the East and West Cottage Streets intersection I feel I am entering Dorchester, but I don’t feel I’m home free yet until I cross Columbia Road at Upham’s Corner.

“I had a conversation recently with an acquaintance and he assured me Grove Hall is part of Dorchester. I’d known of the overlap but I’ve always thought it a part of Roxbury…. It is like comparing platinum to gold, one is somewhat more desirable, but both elements have high intrinsic value worth the investment.”

Fair points, all of them. Here’s our take on the “boundaries:” Historically, Dorchester and Roxbury each were separate towns, not part of the city of Boston. In 1870, the two towns ceased to exist as independent municipalities and were annexed into the City of Boston.

A street atlas from that era reveals the original town boundaries, and here’s a brief description of them.

The town of Dorchester was bounded to the north by a railroad line, the Old Colony Railroad, which lies adjacent to the current- day Southeast Expressway.

That northerly border traveled westerly, across the area that was once wetlands known as South Bay (now landfilled, and the site of the popular shopping center.) Dorchester town’s street border was Magazine Street, which runs now from Mass. Ave to Dudley Street, adjacent to St. Patrick’s church. The south side of Magazine was Dorchester, the opposite side Roxbury. That straight-line border continued to Blue Hill Avenue, with the east side Dorchester, the west Roxbury.

That historic boundary continued up Blue Hill, through Grove Hall to the modern-day American Legion Highway. From there, the Dorchester town border went westerly to Walk Hill Street, southerly along Walk Hill to Harvard Street, and west on Harvard to Cummins Highway, then southerly again to the Neponset River. Historically, all of the neighborhood now called Mattapan was part of the incorporated town of Dorchester. In 1870, Mattapan Square was a real suburban outpost, a station on a rail line that brought people to the country side beyond- Milton, Canton, Sharon.

As the city grew, and the neighborhoods became populated, the federal government established postal stations, with arbitrary decisions about what areas the stations would cover.

Grove Hall was designated “Dorchester 21” (now 02121,) mail addressed to that station was carried to homes on both sides of Blue Hill Ave. That’s how some Roxbury streets have Dorchester postal addresses. Similarly, a large part of Lower Mills is split between two postal stations, Dorchester 02124 and Mattapan 02126.

Our blogger friend has it right: “ In the end does it matter? It is Grove Hall, Boston, after all, another example of parochial hair-splitting. It is like my personal decision to end Dorchester at Andrew Square. That seems to be where the Dot’s magnificence ends and Southie’s begins.”