Back Bay is a textbook example of urban fabric: rectangular blocks, east-west thoroughfares of different scales, north-south cross streets with names in alphabetical order, back alleys running down the middle of the blocks and rows and rows of narrow three-and-four-story party-wall row houses.
Yet, when the fabric of Back Bay reaches Copley Square, it turns into a city of buildings. Both H. H. Richardson's Trinity Church to the east and Charles McKim's Boston Public Library to the west bookend the space of the square not only at a larger scale but with a completely different massing, not the regular repetition of residential structures, but free-standing volumes with distinct iconographic profiles. Facing each other across the square, the two buildings represent, in a way, the choice of city that the United States faced after the Civil war, with the church as an earlier language of public buildings and the library as a harbinger of the architecture that came to dominate urban America for the next half century or so.
Although more embedded in the fabric, you could add the Old South Church to the collection. Also, until 1908, the massive gabled volume of the Museum of Fine Arts on the south side, later replaced by the Copley Plaza. And, of course, after 1977 the 60-story exquisitely restrained reflective volume of Harry Cobb's Hancock Tower.