Short answer:
Part of the downtown-side of Mass Ave – roughly the area between Tremont and Albany Sts. – is the South End. That would include Chester Square and Chester Street.

Long answer:
Looking at the historic maps, the streets are an imperfect way of determining the old boundaries. Roxbury’s northern land area was never a straight line. Roxbury had peninsulas extending north of Mass Ave west of Tremont St (into the Back Bay) and east of Albany St (into South Bay).

On land, the South End extended south past Mass Ave between Tremont and Albany, just to the north of Ball St. You can still see a portion of a boundary stone on Washington St across from the baseball field.

A series of annexations of parcels of land from Roxbury to Boston (starting at Dover St and ending at Ball St), followed by annexation of all of Roxbury in 1868 and land-building into the Back Bay and South Bay blurred the location of the natural boundaries. Social “convenience” (i.e. racism, bias against immigrants, etc) hardened the boundaries to the street boundaries we use today.

According to The City Record and Boston News-Letter, Mass Ave was created from Chester St, Chester Sq, and Chester Park. Based on what I could see on the maps, Chester Sq and Chester St were in Boston (the SE) when Roxbury was still its own city.

I looked at maps 7, 11, and 13 at http://www.discoverroxbury.org/#/historicmaps/4528113868

Information on Mass Ave is at http://bostonhistory.typepad.com/notes_on_the_urban_condit/2005/01/changes_in_bost.html

Are the homes on Mass Ave in Roxbury or the South End? All the homes in this post are on the downtown-side of Mass Ave:

Then and Now: Chester Square

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All of the South End has not always been part of Roxbury. When Boston was connected to the Roxbury by an isthmus of land, part of that isthmus was called Boston Neck, the other part was called Roxbury Neck. The gate between the two towns was at Dover St (now E. Berkeley St).

As Boston grew and began filling in the tidal lands, the cities negotiated land transfers from Roxbury to Boston. As the border moved south from Dover St the tidal lands were filled by Boston, thereby making the newly acquired land the southern end of Boston, a.k.a. the South End. The portion of the Neck that was existing land around Washington St was annexed to Boston, but the area that was swamp and/or water was created to become Boston, not annexed from Roxbury.

Until the 1800s, Roxbury also had peninsulas which jutted into the tidal flats along the Charles River (which became part of the Back Bay) and into South Bay. Those peninsulas, along with Mission Hill, the Longwood area, and parts of the Fenway were all part of Roxbury at the time of Roxbury’s annexation to Boston in 1868.

Most people I know consider Mass Ave as the current line between Roxbury and the South End, even though historically, the division hasn’t been a straight line since it moved south from Dover St.

You can see how Roxbury changed by viewing the historic maps assembled by Discover Roxbury at http://www.discoverroxbury.org/#/historicmaps/4528113868

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Melnea Cass Blvd is a poor demarcation point between Roxbury and the SE. It’s a 20th century road forcefully imposed upon a community that existed since the 17th century. That road was originally designed to be part of the highway system. Hundreds of businesses and homes were destroyed to make way for the highway until area residents succeeded in stopping that project. Many long-time residents on both sides of Melnea Cass consider the area between Melnea Cass and Mass Ave as being Lower Roxbury.

Given that Roxbury has been a town since 1630, various areas were developed over time for different reasons and by different architects and developers, resulting in the variety of architectural styles. Fort Hill once had estate mansions before it was subdivided for rowhouses. Mission Hill was a farming community before being subdivided with rowhouses for factory workers who worked along the Stony Brook. Lower Roxbury also had factories. Some factory owners there built workers housing to attract and keep workers in the area.

(and then there’s Roxbury vs. West Roxbury…)