LOS ANGELES  Jay Leno is putting a big dent in the “checkerboard.”

With one sweeping shift this week, the ailing NBC network reordered the playing field of prime-time television. The introduction of a five-night-a-week program starring Mr. Leno, beginning next fall, was a concession that TV norms cannot continue, at least not at fourth-place NBC.

The programming and viewing habits of the last 50 years  exemplified by the checkerboard of competing programs on the broadcast networks  are being replaced by an Internet-influenced time-shifting model of scheduling. As a result, the very definition of prime time may be changing.

“We do have to continue to rethink what a broadcast network is,” Jeffrey Zucker, the chief executive of NBC Universal, a unit of General Electric, said at an industry conference Monday, hours before the news of Mr. Leno’s new assignment emerged. He warned that if changes were not undertaken, “the broadcast networks will end up like the newspaper business or, worse, like the car companies.” Maybe Mr. Zucker has seen the future; after all, his network has lost 50 percent of its 10 p.m. audience in the last three years.

The announcement of Mr. Leno’s show continues to reverberate on studio lots and executive corridors here, as the Monday-through-Friday “strip” is unprecedented in the modern network television era. NBC framed the decision in terms of competitiveness and cost-effectiveness, because it defuses the risk of Mr. Leno’s move to another network and saves untold millions of dollars a year. But it also reflects the increasing irrelevance of the network schedule.