Imagine if the layout of your local supermarket were constantly in flux. One week, the checkout lanes might be located up front; the next, they'd appear in place of the frozen-food section, which would move to the bread aisle. Until, a few weeks later, the whole thing would unaccountably shift again.

Confusing as that sounds, the same sort of dislocation and disorientation infects network TV schedules each fall. Seeking strategic advantage but risking viewer alienation, network programmers prune, shuffle and juggle their prime-time program lineups. They move beloved favorites to unfamiliar time slots. They move unknown new shows to familiar time periods. They preempt. Within weeks, they do it again.

This week, with the official start of the new TV season, only a few popular programs will be where viewers left them last season. "60 Minutes," for example, will air on the same time and day it has since 1975, Sundays on CBS at 7 p.m. (or whenever the football game ends); "The Wonderful World of Disney" will air opposite on ABC; and "Friends" will once again hold the lead position in NBC's flagging Thursday night lineup.

But these days, such trusty anchors are the minority. Despite boasting to advertisers that they've toned down the tinkering this year, the networks have made more changes to their lineups this fall than any in recent memory.

Almost three of every five series carried on the six broadcast networks this fall will be new shows or returning shows slotted in new time periods--a 60 percent "churn" rate, according to TN Media, an advertising research firm. The biggest revisionists: the WB and UPN networks, which have churned, respectively, 73 and 82 percent of their prime-time schedules.

"There's so much change that it's going to take a few weeks for people to figure out where [programs] are," says Stacey Lynn Koerner, vice president of research at TN. "It's kind of hard to know any more when to be around to catch your favorite shows."

To make matters more confusing, "premiere week" really is neither. Yes, the bulk of network shows will debut this week. But not all programs will be new, and not all new programs will debut at their "regular" times.

Struggling UPN debuted five of its series the week of Aug. 22. Other networks will scatter their premieres through the fall; the new seasons of ABC's "NYPD Blue" and Fox's "X-Files" won't start until the November "sweeps" ratings periods. Viewers who tune in this week hoping to see a new episode of "ER" will instead see the new drama "Third Watch." ("ER" won't be up and running until Sept. 30, by which time "Third Watch" will be appearing Sundays at 8.) Two shows Fox is including in its new fall lineup, "Manchester Prep" and "Malcolm in the Middle," won't be on the air until December at the earliest.

By which time, of course, the mid-season replacement shows will be tumbling forth.

Network executives concede that some of this mucking about is self-defeating. Beset by intense competition--from cable, from VCRs, from the Internet--broadcasters' share of the prime-time audience has been eroding for decades. Moving a show out of its old time period, or to a new night, compels viewers to work to find it; many simply don't bother. What's more, the changes force the networks to spend valuable promotional air time educating viewers about where their old standbys have gone.

"With all the challenges we face to get viewers to come to the network, we're clearly making it harder on ourselves," acknowledges Preston Beckman, NBC's chief program planner and scheduler.

But churn isn't about viewer convenience. It's basically about viewer manipulation.

Although they're aware that changing a show's time or day can lead to defections, program executives are more concerned about increasing their audiences at a given hour, rather than for a given show. They know that loyal viewers will usually follow a favored program to a new spot. Thus the key issue is whether a change helps the network's overall strategic position. As CBS scheduling chief Kelly Kahl puts it: "A new time period rarely helps a show. But it can help the time slot."

Improving the ratings at a particular hour helps boost a network's advertising revenue. And it has an additional benefit: It enables a network to increase its "audience flow"--the idea being that viewers of one show will hang around to watch the next one. And the next. If everything clicks, the strategy can give a network momentum that carries it through the week.

This year, all six networks are scattering more of their returning shows across their schedules in hopes of luring viewers to new nights and time periods. In all, 33 of the 52 returning series will air at new times--a 50 percent increase over the last two seasons.

The trick, of course, is to patch a hole on one night without making a new one somewhere else.

Now You See It . . .

Consider the journeys of one show, "3rd Rock From the Sun."

The NBC sitcom premiered in January 1996, and within weeks of its debut on Tuesdays, it was already on the move. First, NBC moved it back a half-hour, to an 8 p.m. start. In its second season, it moved to Sunday nights, where it became a Top 10 hit. In its third season, it bounced from Wednesdays to Tuesdays and then back again to Wednesdays. This fall, it will be back where it started--Tuesdays at 8:30.

Grand total: In 3 1/2 seasons, "3rd Rock" has had eight regular time slots (and has appeared in about a dozen irregular "fill-in" slots, according to its producer, the Carsey-Werner Co.). With more than a trace of irritation, John Lithgow, the program's star, has called his show "the wandering Jew" of network television.

Perhaps as a result, "3rd Rock" has slipped badly in the ratings, falling out of the top 30 in its first two years to 73rd place overall last season. The audience losses came despite Emmy-winning performances by Lithgow and co-star Kristen Johnston.

Still, "3rd Rock's" losses arguably have been NBC's gains. By moving the popular show up against ABC's Tuesday and Wednesday sitcoms, it blunted ABC's growing momentum on those nights, and helped NBC sustain a few shows ("Just Shoot Me," "News Radio") that might have died earlier without its help. On the other hand, NBC is still trying to rebuild its Sunday lineup, which has never fully recovered from the loss of "3rd Rock."

Putting the "right" show in the "right" time slot is, of course, critical to a series's success or failure. Network corridors are filled with lore about time changes that brought hits low, or rocketed also-rans to glory. "60 Minutes"--the most-watched program in the history of prime-time TV--was once ranked 101st of 103 series when it aired on Tuesday nights; its move to Sundays in 1975 transformed it into a staple of the Nielsen ranking's top 10. Conversely, CBS sealed "Murphy Brown's" fate in 1997 by moving it from Monday to Wednesday. And Fox created a pop-culture phenomenon in 1997 by moving the already popular "X-Files" from Friday to Sunday.

For years, the cushiest spot on TV has been the Thursday 9:30 slot on NBC, before "ER" and after "Seinfeld" (and lately "Frasier"). NBC has used this "hammock" to launch or boost a succession of sitcoms, including "Frasier" and "Mad About You," both of which went forth to build Tuesday and Wednesday franchises.

The Thursday sweet spot has even propelled the likes of "Suddenly Susan" and "Veronica's Closet," neither of which seems likely to have succeeded without the push. Ratings for both these shows have plunged so dramatically since moving off Thursdays that CBS President Les Moonves this spring publicly derided NBC's new Monday lineup (featuring "Susan" and "Veronica") as the network's "dumping ground for failed Thursday night comedies."

Who's On First

Network schedule-setting can be a complicated undertaking--part science, part black art. Programmers work for months to assemble their fall and mid-season lineups. The process typically involves input from dozens of executives representing the network's production, sales, research, affiliate relations and promotion departments.

"There are a million nuances to this," says NBC's Beckman. "The historical and institutional knowledge is the most valuable thing we bring to this job. We've all made mistakes. But it's like a lab rat. If you don't learn something from being burned, you're an idiot."

There's a fair amount of guesswork involved, too, given that each network has only a general idea what its rivals are plotting and counterplotting before the fall lineups are revealed to advertisers and the media each May. This season, the youth-oriented WB network seems to have made a good guess by moving the college-age drama "Felicity" from its original Tuesday slot to Sundays at 8, when it will face less competition for its core audience of teenage girls and young women.

Some shows get on the schedule not solely as a result of creative merit or strategic importance but because of network self-interest or back-scratching with powerful producers.

Does anyone remember "Love and War," a 1992 sitcom on CBS? It was Diane English's reward for making a hit out of "Murphy Brown." It got a choice time slot, right after "Murphy" on Mondays. To make way for "Love," CBS moved "Designing Women," created by Linda Bloodworth-Thomason, into the vast wasteland of Friday nights, thereby speeding that show's demise. No hard feelings, though: CBS had already handed Bloodworth-Thomason her own deal that ensured favorable time slots for her hatchlings, including the failed "Hearts Afire."

This fall, Fox made room on its Tuesday schedule for half-hour repeats of the preceding night's "Ally McBeal" by bumping off Eddie Murphy's "The PJs." Although "PJs" had performed well enough for Fox (and may return at mid-year), Fox owns "Ally" and wanted the time for the rerun to improve its chances of being sold into syndication. ABC pulled a similar stunt, putting a new comedy that it partially owns, "Oh Grow Up," in the spot after the popular "Drew Carey Show."

And "Veronica's Closet"? It wasn't just series star Kirstie Alley's labors on NBC's "Cheers" that ensured the show's existence on NBC. "Veronica's Closet" happens to come from an outfit called Bright/Kaufman/Crane Productions. Call it payback. BKC happened to have created an earlier series for NBC, called "Friends."