The building will be thoroughly modern. The midtown Modern, scheduled to open in late November after being closed for two and a half years, is 630,000 square feet of straight walls, floors and ceilings with no obtrusive columns or dead-end hallways. It is a building with "a harmonic precision," Mr. Lowry said.

But simple architecture is not always simple. Making a precise, rectilinear museum in a city that refuses to bend very much requires lots of logistical contortions. To cite a small example, the builders had to cut a notch out of one of museum's facades to preserve the view of St. Thomas Church from 54th Street. And in New York City many unions are involved, each with its own schedule. That makes a precisionist structure, where quarter-inch goofs can throw off the whole thing, extremely tricky to build.

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The first and biggest obstacle to the Modern's seamlessness was the 54-story residential tower between the old and the new parts of the museum. The goal was to allow visitors to pass from old to new without ever knowing they had left the museum, Mr. Lowry said. But the tower, with its 248 residents, could not be razed. So the Modern got permission to "slice through the tower," Mr. Lowry said, "to penetrate the four feet of concrete that supported the tower." To insure that the tower, pierced by the Modern's passages and escalators, would not collapse, steel braces had to be installed in the lower nine floors.