Charles Eames, who changed the way the 20th century sat down, died of a heart attack Monday in his native St. Louis.He was 71.

One of the two or three greatest mid-century modern designers, Mr. Eames was also an architect, movie-maker, photographer and educator. More than that, he was a man who profoundly altered the way people learned through looking.

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Mr. Eames was in St. Louis to make a film about Monet and to consult on an arthitectural project. His studio is in Venice, Calif, and his home in Pacific Palisades, Calif.

Mr. Eames collaborated with his wife, Ray, for 40-odd years on projects that began with an innovative chair design, the first molded plastic chair that could be mass produced. Today more than 6 million people could all sit down on Eames chairs in airport terminals, lobbies, offices, and private homes. More than $500 million worth of Eames-designed chairs have been sold.

The Eames' molded plywood "potato chip chair" was the first piece of modern furniture that average people could afford in the 1940s. The plastic shell chair has become ubiquitous. And the $1,604 leather-and-rosewood lounge chair and ottoman is widely considered the ultimate in expensive comfort. Their work completely revolutionized the way furniture is made and looks in the 20th centry. Their designs are sculptural, streamlined and sensuous. The curving shapes echo and cuddle the human body. The chairs' unequivocal 20th century shapes are technologically sophisticated, yet they are based on the consold human form.

In 1946, George Nelson, then the design director of the Herman Miller Company, brought Mr. Eames to that pioneer manufacturer of contemporary furniture. Miller still manufactures Eames chairs, including some variations not yet in the salesrooms.

At the height of the popularity of his chairs, Mr. Eames, a man of long and lonely thoughts, began to concentrate more on concepts than on products. In 1953, Nelson called him in on a project to teach University of Georgia students about art. Nelson worked out the first multimedia projector approach to presentations. Mr. Eames developed from this an obsession with teaching people more and faster - the three-ring circus or computerized approach to learning.

Mr. Eames said, "People have accused me of using several projectors so I could make them look at more of my slides at once."

He believed that people were capable of learning many things at one time. "You can drive, think about your own car, cope with others, and be thinking about something else at the same time," he said.

One of the most successful of his multimedia programs was a 1958 collaboration with Nelson for the American pavilion at the Moscow World Fair.

The 50 or so Eames movies - a remarkable mixture of profundity and naivete - are counted as works of art in themselves. Enthusiasm for them is a cult Mr. Eames always said that he and his wife took up film making when a friend left in their car a 16mm movie projector.

Their film, "Powers of Ten," is a serious effort to explain mathematical concepts visually. He had just recently expanded and revised the film. The early "Powers" is shown everyday at the Smithsonian's Air and Space Museum. A film about Charles and Ray Eames, "An Eames Celebration," was shown on public television in 1975.

Mr. Eames had always been interested in photography. His father died when Mr. Eames was quite young leaving him a great store of antique photographic equipment. Mr. Eames used to joke that he was 30 "before I knew that film came in rolls, not just wet emulsion."

He and his wife wore cameras the way some people wear belts. They would take your picture almost before they shook your hand. When you'd ask them a question, they'd show you a picture. Instead of writing or drawing proposals, they would present their clients with elaborate films or slide programs.

His "time line" concept was a sort of chart that would go all the way around a room. Each year or era would be represented by a photograph with a brief caption. He developed the time lines for many ideas: one on mathematics; "The World of Jefferson and Adams," a Bicentennial exhibit that was shown all over the world; and one, distributed privately, tracing the history of Herman Miller and modern design.

Mr. Eames never quite gave up architecture. He and Kevin Roche once were selected by the Commerce Department to design a great aquarium for Washington. The design was accepted, to loud critical acclaim, but was never built.

Mr. Eames had worked as a consultant to the Metropolitan Museum of Art, when it was planning a great learning center. He also consulted with the Library of Congress about ways to organize information. He was much in demand on campuses for speeches and was awarded almost a dozen honorary degrees. He served on many committees, including the National Endowment for the Arts.

Mr. Eames first met his wife at the Cranbrook Academy of Art in Bloomfield Hills, Mich. Mr. Eames has studied architecture in St. Louis and worked for an architectural firm. But he left school in a dispute over his championship of the avant garde architect of the period, Frank Lloyd Wright. Eliel Saarinen, president of Crambrook, gave him a fellowship. Ray Kaiser, a painting student there, and Charles Eames fell in love, and began their private and public collaboration.

Mr. Eames' class of 1935 included many other artists and designers who were to exert profound influence on American design and concepts - Florence Knoll, the furniture designer; Harry Weese, who designed the Washington Metro; Eero Saarinen, architect of Dulles Airport, and Harry Bertoia, a sculptor who also designed chairs.

Mr. Eames and Eero Saarinen - with help from Ray Kaiser - won a 1940 Museum of Modern Art Competition for an organic chair design. But it was wartime, and the chair was not immediately produced. In 1941, Charles and Ray Eames married and went to California, where they had a government grant to work on prosthetic devices, splints and molded parts for the Navy. Not incidently, the work applied to their molded chair.

During the day he worked as a Metro-Goldwyn-Mayer movie set designer, while she experimented with molding techniques. They both worked most of the night. Their workship was their small apartment.

They smuggled wood up the elevator and used the oven of the stove to cure the rubber shock mounts for the wood. At least once the oven blew up. They produced the chair themselves until it was sold in 1946 to the Herman Miller Co.

There are legends about the way Mr. Eames practiced "less is more in conversation." In 1956, he gave to the movie producer Billy Wilder the first of the leather Eames lounge chairs as a gift.When he brought it to Wilder, he said, "Take chair."

The American Institute of Architects recently gave Charles and Ray Eames the AIA 25-year award for their innovative house - built out of stock steel and glass prefabricated factory parts - in Pacific Palisades. Earlier, he won the AIA's craftsmanship medal for furniture, and in 1972 its industrial arts medal.

Ray Eames said she and her husband spent little time in their house. "We have breakfast at home, leave for the workship about 10 a.m., and stay until 8 or 9 at night." The house, so spare to begin with, eventually was covered, like their wonderful workshop, a converted garage, with their collections.

"Toccata for Toy Trains," one of their earliest movies, best shows the Eames feeling about toys. They collected shells, fish (the aquarium project), marionettes, buttons, tops - what have you. They justified their mania by calling them "props," their collection rested upon every flat surface and hung from the ceiling in their workshop.

They made toys as well. The 1952 Eames House of Cards was a series of notched cards with macro lens photographs of buttons, shells, marionettes, and toys, in brilliant colors. In 1957, they built a perpetual motion machine called a Solar Energy Do Nothing Machine for Alcoa Aluminum, and in 1951, The Toy, a set of movable triangles.

Perhaps the toys are the most important clue to Charles Eames. For he made work out of pleaure, pleasure out of work, and an art of life.

Besides his wife, Mr. Eames leaves a daughter (from a previous marriage), Lucia Demetrios, of San Francisco, five grandchildren, and a sister, Adele Frank, of Jackson, Miss.