Louis Kahn was considered a pretty good modern architect in 1945 when Anne Griswold Tyng went to work in his office, then located in the Evening Bulletin building across from Philadelphia's City Hall. By the time they parted company two decades later, Kahn was revered for liberating architecture from its Bauhaus straitjacket and Tyng was known, if she was known at all, as his mistress.

Had they embarked on their storied collaboration today, one imagines Tyng sharing the credit for their breakthrough work, especially the Yale Art Gallery and the Trenton Bath House. They would call their firm something like Kahn & Tyng. And Tyng would surely merit more than the few throwaway lines she gets in today's rapidly expanding library of Kahn books.

The past is past. But, fortunately, enough has changed in the profession that, at 90, Tyng is about to enjoy the first retrospective devoted to her architecture. While the show, "Anne Tyng: Inhabiting Geometry," which opened Thursday at the Institute of Contemporary Art, doesn't completely make up for a lifetime of slights, it goes a long way toward explaining the influence she had on what we still, not quite accurately, call "Kahn's work."

Because architecture is such a collaborative art form, establishing attribution is often difficult, no matter which genders are involved. As big and complicated as buildings are, we like to pretend that they spring full-blown from a single mind. But you only have to examine the evolution of Tyng's sketches in the ICA exhibit to understand that a modern masterpiece such as the Yale gallery wouldn't exist without her contribution.

Historians now agree that Tyng was the first in Kahn's struggling office to become enthralled by space-frame trusses, the first to speak rapturously about incorporating their superstrong tetrahedrons - pyramid shapes - into a building's bones. Kahn was smitten by his 25-year-old assistant's theories, as well as her drop-dead blond looks. Their work became inseparable from their long, passionate affair.

Those tetrahedrons, along with Tyng's other geometric obsessions, eventually found their way into the Yale gallery design. Its triangular ceiling grid, which was also an early effort at providing easy access to electrical and ventilation equipment, remains one of the museum's most distinctive features - so much so, it appears to have inspired the ceiling in Diller Scofidio & Renfro's just-released proposal for the Broad Art Foundation on Grand Avenue in Los Angeles.

The Yale gallery, completed in 1953, was a roaring success, and it finally established the talented, but unlucky, Kahn as an important architect. Kahn continued to incorporate geometric shapes into his work in various ways. They became a hallmark and helped give his designs a powerful, almost classical, monumentality and presence that distinguished his architecture from the thin-skinned buildings of the then-reigning International Style.

People who know Kahn's work will recognize many of the drawings and models that guest curators William Whitaker and Srdjan Weiss have included in the ICA show. These elements have been appearing for years in books about Kahn, always attributed to Kahn. The retrospective restores credit to Tyng.

Tyng, who is still ravishingly beautiful, is more modest about her accomplishments. "Geometry belongs to everybody," she insisted as she watched the exhibit being installed this week. She delighted in seeing models from decades ago, like the famous, zigzagging City Tower, whose overall shape echoes the structure of its tetrahedron struts. Still impressive today, it was never built, though it strongly influenced designers and its DNA wound its way into projects such as Norman Foster's Hearst Tower in New York. A young Robert Venturi designed City Tower's base.

Most of the objects on display at the ICA came from the basement of Tyng's Center City home, a typical rowhouse that she reconfigured to demonstrate her ideas about geometric forms. After she sold the house - coincidentally, to the ICA's senior curator, Ingrid Schaffner, who calls it a "work of art" - Tyng donated the trove to the Architectural Archives at the University of Pennsylvania, where Whitaker is curator. The ICA show grew out of the effort to catalog the material.

Whitaker believes the first years of Tyng's collaboration with Kahn were "an incredibly powerful moment, when ideas were going back and forth, and these shaped the work of the '50s."

In 1953, Tyng left Kahn's office for Rome on the pretext of studying architecture. In actuality, she fled there to give birth to their daughter, Alexandra Tyng.

She stayed away 18 months, but Kahn's letters, which she published in 1997, suggest Tyng never stopped contributing. "He definitely learned something from her," Whitaker says.

When she returned to the office, she was drafted to help design the changing rooms for a community swimming pool outside Trenton. Tyng immediately shifted the floor plan from a shoe box to a cross shape, establishing the arrangement of five cubes that made it one of the most transformational works of 20th-century architecture. She was also the one who developed the hollow columns that simultaneously hold the roof and serve as entrances to the rooms. But you won't find Tyng's name on the bathhouse project, either.

The exhibit at the ICA offers the chance to see Tyng's ideas before they were subsumed by Kahn's own strong vision. Also on display are the handful of projects she developed as an individual. The show includes a model of her parents' house on the Eastern Shore of Maryland, another demonstration by Tyng to show the strength of tetrahedrons, as well as several models for unbuilt projects.

You can't look at this show without wondering how Tyng's career might have developed if women had been treated differently back then. From the time she was admitted to Harvard's architecture school, in the first class to accept women, it was clear her talent was immense. But after graduation, she discovered that New York firms wouldn't hire her; they considered it unseemly to have women in the office.

Even Kahn could minimize her contribution. After their affair ended in 1960 (when Kahn became involved with landscape architect Harriet Pattison), he neglected to list Tyng on the credit label when the City Tower model was included in a show at the Museum of Modern Art.

Interestingly, as Weiss, the guest curator, notes, the geometric thinking that Tyng advocated now forms the core of contemporary architecture. But the extensive computations that Tyng scrawled alongside her pencil drawings are now done in milliseconds by computers.

For the show, Weiss created five oversize, multisided geometric forms, based on the Platonic solids, which Tyng used as a basis for her ideas. He also hung a spiraling double helix from the ceiling that seems to dip and whir over the proceedings.

Several of the geometric forms are almost a full story high, big enough to walk through. When Tyng saw them, that's exactly what she did. It was the first time.

Changing Skyline: If You Go

"Anne Tyng: Inhabiting Geometry" runs through March 20 at the Institute of Contemporary Art, 118 S. 36th St. ICA is open from 11 a.m. to 8 p.m. Wednesday, 11 a.m. to 6 p.m. Thursday and Friday, and 11 a.m. to 5 p.m. Saturday and Sunday. Admission is free. For more information, call 215-898-7108.EndText