"Sever Hall is Janus-like," dividing Harvard Yard into past and present. The massive building's brick exterior harks back to Federal style and its new shape looks forward to modernism.

"On the left, there's a massive gray wood fence, punctuated by a mailbox. On the right, the side of a brown-shingled Victorian house. "Here you have the history of modern American architecture," Leslie Humm Cormier says as she steps out onto the front steps of the Cronkhite Center on Ash Street. She waves her hand to include both sights.

Cormier, an architectural historian, has offered to take me on a walk to explain her Radcliffe Seminar course, American Architecture in Situ.

The house on the left -- I have to take it on faith that it's a house, because all you can see is the fence, which wraps around a corner lot -- is Philip Johnson's creation of 1941. On the right, according to Cormier, is the most important house in American architecture, the Stoughton House, built by H.H. Richardson in 1882.

She will explain further. Cormier is in the business of teaching people how to "read the street." She wants people to react to buildings and talk about them. Most people don't see what's around them, she thinks, mainly because no one's ever taught them how.

A detail of Sever Hall. "At Sever Hall, [H.H.] Richardson is re-inventing American architecture," says Leslie Humm Cormier.

The thing that complicates the architectural historian's job, Cormier says, is that architecture is the only art that's functional. And it's so big.

Architecture is so big and functional that it's hard to pin down the moment when one style changes into another.

The Philip Johnson house may be "modern," but from the side, the Stoughton House looks to me like one more 19th century pile of wood.

We walk around to the front of the Stoughton House on Brattle Street and stand in the driveway. To my surprise, it is magnificent, in fact an astonishing house. H.H. Richardson has created a romantic, sinuous structure, with a turret that seems to bulge out of the shingles, a round tower, and a gallery stretching across the facade. Nothing about the two-story house is symmetrical.

It's huge, but it's not set very far back from the street. Its exuberance proclaims it to be simpatico rather than domineering, a potbellied gent with a watch-chain across his ample vest and a half-chewed cigar poking out of his mouth -- someone who reads the Racing Form rather than The Wall Street Journal. Dark brown shingles cover all of it, a natural skin enclosing the building, which seems to grow out of the earth.

"Here H.H. Richardson is really creating the American idiom," says Cormier. "He's thrown off his European antecedents. Using native materials, he's created something completely new. It's very modern: the windows are elongated; they don't break the surface. It's very American: the shingles are a simple material given a classy treatment. The tower grows up out of the roof line; it's suppressed for the sake of continuity. Any element is secondary to the overall effect. It's a stretched skin over a volumetric treatment of space."

She continues: "Mies van der Rohe, who trained Philip Johnson, said, 'Architecture is almost nothing,' " Cormier declares.

In Radcliffe Yard, no individual building makes an assertive statement, but the peaceful, pleasant oval makes an "academical village" -- a phrase used by Thomas Jefferson to describe his design at the University of Virginia.

So what does the Richardson house have in common with the Philip Johnson house?

This is the moment that makes the walk with Cormier a discovery rather than a stroll.

Cormier explains that Richardson uses elemental forms, the cube and the cylinder, to enclose space. He respects the surface continuity. The "Shingle Style" house on Brattle Street is not symmetrical, but it has unity. And Philip Johnson uses the same elements.

In one of those "aha!" moments, I understand how the Philip Johnson house is a continuation of the thinking that produced the Richardson house. This perambulation is more than a walking tour of Cambridge. Like Cormier's course, it is a microcosm for the study of American styles.

"Where else in 60 years, in 60 paces, can you have this kind of transition?" she enthuses.

Cormier crosses the street to Radcliffe Yard.

The Yard is built in the Federal-cum-Greek Revival style that is the quintessence of Ivy League architecture, she explains. It is both an enclave and an ensemble. No individual building makes an assertive statement, but the peaceful, pleasant oval makes an "academical village," as Jefferson said of his design at the University of Virginia. Planned by McKim, Mead and White, built about 1897, Radcliffe Yard derives from a long train of Neo-Classical thought. The red brick buildings in Radcliffe Yard, especially the immense colonnade of the library, resonate, speaking of a continuity going back to Ancient Rome and Greece.

The red brick buildings in Radcliffe Yard, especially the immense colonnade of the library, resonate, speaking of a continuity going back to Ancient Rome and Greece.

Cormier's lesson is not finished. Think about Richardson's Sever Hall, built about the same time as Radcliffe Yard, she says. It is monumental, yet seems organic. It speaks of power, public and private.

"At Sever Hall, Richardson is re-inventing American architecture," says Cormier. "Sever Hall is Janus- like," dividing Harvard Yard into past and present. The massive building's brick exterior harks back to Federal style and its new shape looks forward to modernism.

Harvard has always vacillated between the Parthenon and Stonehenge, the two poles of architecture, says Cormier. Yet Harvard architecture has never been merely "pleasurable." Rather, Cormier said, "The planners have asked themselves, 'How can we monumentalize the moment?' They have chosen the example that speaks to the moment in architecture. Each new building has significance in time and place. The building is background no longer."

After a walk with Cormier, we, too, find the buildings stepping onto center stage, eloquent representatives of their era and the thoughts of their creators. We enter into a conversation with them. Through her, we have begun to "read the street."