"New York is always grown up . . . assured. Chicago is always adolescent . . . vigorous. New York establishes, verifies, knows. Chicago hunts, tries, hopes. New York is the boundary, the front door. Chicago is the middle surrounded by America."

This is how Laurence Booth, an architect from Chicago, encapsulates the differences between the nation's "first" and "second" cities, the one more than 350 years old, the other less than half that; the one a tight island between narrow rivers, the other an endless plain facing a great lake; the one somehow securely tied to Europe and the past despite a daunting stretch of ocean, the other somehow culturally rootless in the middle of a continent despite being a mighty hub of transportation.

And yet the two cities are more alike in image and reality than any listing of their differences would allow. The reason for this is fairly simple: As competing centers of American corporate and financial life, the two cities have played very nearly equal roles in the continuing development of that most American of building forms, the skyscraper.

"Chicago and New York: Architectural Interactions," an exhibition on view through Jan. 6 at the Octagon (1799 New York Ave. NW), sets out to explore the differences and similarities between cities "often thought to be the two great superpowers of American architecture," in the words of John Zukowsky, curator of architecture at the Art Institute of Chicago, which organized the exhibition in collaboration with the New York Historical Society.

It is an ambitious, fascinating, difficult exhibition. The ambition is built into the concept of an undertaking designed to compare the physical presences, and to some degree to analyze the forces that shaped them, of two huge cities over the course of nearly two centuries. The fascination and the difficulty, from a viewer's standpoint, come from trying to piece together the limited amount of information on the walls.

Architectural exhibitions always suffer this handicap. Even a presentation of complete sets of plans, elevations, renderings, models and photographs of a single project cannot truly rival the three-dimensional experience of the actual building. The difficulty is compounded about a millionfold when the subject is a city, much less two of them.

There are 70 original objects in this show, supplemented by extensive texts containing photographs of key buildings not elsewhere represented. Obviously the account it gives of the vast subject, even when catalogue essays by Zukowsky, David Van Zanten and Carol Herselle are added in, is episodic. But, as I said, it is fascinating, and worthwhile.

Many of the drawings and plans are of surpassing interest in themselves, and often beautiful, too. Anyone wondering why architectural decoration isn't what it used to be, even in a new age of self-conscious ornamentalism, will get a clue from a print of a drawing for the ornamental details of Dankmar Adler's and Louis Sullivan's great 1889 Auditorium Building in Chicago. It isn't simply that a designer of Sullivan's talent does not come along frequently; it is also the degree of control the architect was able to exercise. A handwritten message in between the sinuous designs instructs the craftsmen as follows:

"This design is to be applied to all ornamental arches in hotel lobby and cafe . . . the pattern is to make a complete and special finish at corbels and at spring line of the arches -- no mutilation of pattern for the purpose of making a fit will be permitted."

Some of the "elementary interactions" referred to in the first section of the show, involving work by Chicago architects in New York and vice versa, are among the more famous moments in American architecture: the participation of eastern firms such as McKim, Mead and White in the World's Columbian Exposition in Chicago in 1893, crucial in the choice of a neoclassical style for all the buildings in the "Great White City"; Chicagoan Daniel Burnham's 1902 design for the Flatiron Building in New York, a slender skyscraper that critic Paul Goldberger observed "is, in a sense, what every Manhattan tower aspires to but few have ever been able to achieve"; Chicagoan Ernest Graham's 1912 design for the massive Equitable Building in New York, which caused the passage in 1916 of the country's first zoning ordinance to define the location, shape and height of tall office structures; and the postwar dominance, in both cities, of the International Style, pioneered in Chicago by Ludwig Mies van der Rohe (who also designed the 1958 Seagram Building in New York) and followed through in both cities by Skidmore, Owings and Merrill and a host of lesser firms.

The exhibit focuses some attention on the differing residential patterns that developed in each city from the mid-19th century on, but here, as elsewhere, differences became less important as time went on and Chicago increasingly turned to the high-rise apartment blocks long favored in New York. The most dramatic difference in the image of the two cities -- the way they meet their respective waterfronts -- is mainly due to Burnham's and Edward Bennett's famous 1908 Chicago plan.

There are two stupendous manifestations of this plan in the show -- a map of the proposed realignment of Chicago streets and avenues and a vast three-dimensional rendering. The most enduring positive impact of the plan was the creation of that great stretch of open, public space along the lakefront and the massing of towers behind it. (Burnham opposed the towers, obviously without effect.) More than anything else, this gives Chicago its special, open character, especially when contrasted to the rich texture and pyramidal massing of Manhattan's skyscrapers and the despoilation (only now being corrected in spots) of the island's waterfronts.

Underlying forces that drove development in both cities -- notably the creation of complex, multilayered transportation systems and the existence of economic and technological conditions favoring the construction of tall buildings -- are analyzed at some length in Van Zanten's and Krinsky's essays. In these two respects (most importantly the latter), Chicago and New York unquestionably have acted as the most powerful models of city development our century has seen, emulated in cities around the globe from Hong Kong to Cairo to Houston and Atlanta.

The question is, to what extent is this a good model? This issue is not about style, although it should be noted that in terms of both style and urban design there have been healthy changes in the design of skyscrapers in recent years. (Excellent examples are included in the show and the catalogue: Chicagoan Helmut Jahn's proposal for an office building on Lexington Avenue in New York, and the handsome structure at 333 Wacker Dr., Chicago, designed by the New York firm of Kohn, Pederson, Fox.) The issue is the quality of the urban environment produced by a high-rise commercial core developed almost entirely according to the dictates of speculation in spiraling land prices.

This question occurs with particular poignancy in Washington, a low-rise city whose office buildings, if widely lacking in architectural distinction (and totally lacking the excitement of height), nonetheless are forced to follow at least a few elementary rules -- in many cases very elementary -- of urban decency. It is no accident, after all, that our streets are sunnier, and that trees grow better here. If Washington is a duller place for all of this, I'm convinced that in the long run the city will be better off.