WHAT IS IT that makes a diner a diner? Lots of stainless steel and Formica, you might say. Squeeky chrome-rimmed counter stools and Naugahyde-covered booths. A 24-hour menu heavy on beef and potatoes, breakfast served anytime.

But Gene Wilkes Jr. knows better.

"There's a big difference between a diner and a restaurant," he says, sitting in a rear booth of the Bethesda Tastee, which his father owns and he manages. "A restaurant is a place where you go to eat. A diner is a place you go to relax. It has a certain atmosphere. We have people who are here every day, who eat every meal here. People depend on this place."

Take Samuel Waters, across the aisle. The white-haired retired government worker has just finished his lunch of meat loaf, peas, carrots and mashed potatoes with lots of gravy -- one of Tastee's specials today (there are four). He pushes his plate aside, and will now read two newspapers cover-to-cover, he says, before going back out into the world. He's unimpressed that someone wants to interview him. "Nothing to say," he says, glancing at the front of the New York Times sports section. "Been coming here since before you were born, I bet."

The Bethesda Tastee has been in business since 1939 -- at its present location since 1942 -- and everything about it is pleasantly old, in stark contrast to the ultra-modern hotels and office buildings that have gone up around it. In the world of the diner, there's a saying that time stands still. And people, as Wilkes says, "are just people."

Diner conversations are as varied and uninhibited as you'll find anywhere. On this Saturday afternoon, two preppy types in their 30s are discussing local politics ("What we've got is a banana republic disguised as a city," one says, while the other shakes his head). Beyond them, a young group, taking up two booths, has several conversations going at once. One seems to be about the Grateful Dead, another the Redskins. Suddenly, a booming voice says, "Which is exactly what made 'Night of the Living Dead' such a classic piece of filmmaking."

In the world of the diner, all of this is standard operating procedure. As is the fact that for the last 25 minutes the juke box has been playing one song over and over, a song called "Bad Boy" by Tex Rubinowitz.

"That happened a few weeks ago, too," Wilkes says, unconcerned, "where the juke box gets stuck. If someone wants to play another song, it'll play. But until someone does, we'll keep hearing the same song over and over."

Ah.

As in most diners, there is a Tri-Vue wall juke box at each booth, so I feel in my pockets for a quarter. But without luck.

"One of the big appeals of the diner is that it's old-fashioned," says Wilkes, who's only 24 himself, "and for the most part, the people who come here have basic, hard- working values."

When you begin to understand the world of the diner, you no longer ask questions like "What makes a diner a diner?" And you're not surprised that so many people who work in diners have no ambition to do anything else.

"I quit once," says Jannet ("that's with two 'n's") Emery, who has worked at the Tastee 29 diner in Fairfax for more than 10 years. "But that was over a disagreement, and I came back. I don't have any plans to leave again . . . Diners make you feel comfortable. Most of the people in this place are on a first-name basis."

And people are a diner's main reward, she says, recounting a story of some recent visitors from Kentucky, who tipped her $7 on an $8 tab. "They told me they'd never had a waitress smile at them like that before. That makes you feel good."

Dressed in Levis and a blue, lace-frill tank top with a Harley Davidson insignia, Emery is working alone this evening at the Tastee 29, which has the sleek, classic, silvery appearance of a '40s locomotive car. From the looks of things, though, you wouldn't know it, as several of the regulars go behind the counter and serve themselves.

Like many first-timers, I make the mistake of asking her what those letters above the door to the kitchen mean: "YCJCYAQFTJB."

She tells me: "Your Curiosity Just Cost You A Quarter For The Juke Box."

I give it to her, and she plays 242.

"Conway," she says.

Emery is waiting for "Pops" -- an older man who shows up in the evenings to bus tables, though he isn't an employee.

"We take care of him," she says. "We feed him . . . A lot of people don't have anybody else except the diner."

Indeed, diners remain a beacon to those people in search of a clean, well-lighted place any time of the day or night. A place where the food is real, but cheap, and there are usually plenty of other people. In an era of fast food and ever-changing architecture, diners defy time and trends. A restaurant may have style, but a diner has personality.

But the diner has taken some hard knocks since its heydey in the '40s, when there were an estimated 10,000 in the country; there now are fewer than 3,000. In the District, there once were four diners; now there are none fitting the traditional mold.

In the suburbs, though, there are six, and although one -- the Tastee in Silver Spring -- seems in danger of closing down, a new diner is being planned for Washington, the first in the District since the Dee Cee diner closed its doors on Vermont Avenue in 1963. Developer Jeffrey Gildenhorn, who has enlisted the oldest diner manufacturer in the country, Kullman Industries of New Jersey, to build his, says he sees a resurgence in the diner's popularity. The diner at 5532 Connecticut Ave. NW will be a 66-seat 1945 model diner with a 1949 Coca-Cola machine with 6 1/2-ounce bottles and a 1950s Wurlitzer. Gildenhorn says he expects to open the diner in April.

"People want this kind of thing. I think the diners of today will replace the McDonald's and Burger Kings of tomorrow," he says.

Or, in the words of Nick Markopoulos, owner of Nick's Diner in Wheaton: "People can only take so much fast food."

In fact, diners now seem to have two things going for them, whereas 30 years ago they had only one: The basic appeal of a no-frills eatery remains, but now there's the added lure of nostalgia -- the diner having become a part of Americana. (Even the Smithsonian Institute considered restoring what was once the Melrose Diner in Trenton, N.J., for a National Museum of American History exhibit, until funding faltered.).

The popularity of the diner is usually traced back to New England in the 1870s, where horsedrawn lunch wagons served homemade sandwiches and coffee. In 1887, the first "walk-in" lunch wagon came along, in Massachusetts, and the first lunch wagon patent followed four years later; it included a space with stools, where customers could sit and eat, and a tiny one-man kitchen. These early forms of the diner quickly became social centers, and eventually began staying open 24 hours a day. The popularity of such eateries increased in the early 1900s -- largely in the northeastern United States -- as entrepreneurs bought up abandoned street cars for as little as $10 and turned them into eateries. It wasn't until the 1930s, though, that diners took on the classic look of locomotive cars and many of the now-traditional diner features emerged -- Formica, stainless steel, glass-block walls.

Washington's Dee Cee diner opened in 1930 and for the next 33 years was frequented by Redskins, Senators (not to mention senators), numerous celebrities and plenty of just plain folks. But despite its tremendous popularity, the Dee Cee was forced to close when rent in the city rose too high. The same thing happened in many other northeastern urban areas.

The needs the diner fills, though, never go away.

It's 2:15 a.m. at Bob and Edith's diner in Arlington as a group of bar-weary men and women in their mid-20s arrive. One is singing Michael Jackson's "Bad" -- badly. In the world of the diner, this is the night shift -- after the bars close -- and the patrons in Bob and Edith's don't pay these four much mind. An older man at the counter, sipping coffee and reading a magazine, doesn't seem to notice them.

Since most diners don't close, they must be cleaned while customers are eating. A waitress is sweeping the floor as this group goes to a booth, passing by a Dallas Cowboys poster (owner Bob Bolton makes no bones about being an ardent Cowboys fan). One of them says something about her being a witch -- a reference, apparently, to the broom. "Yes," she says, "but a good witch."

"Ha, ha," says the one who would be Michael Jackson.

His name, it turns out, is Russell Marks, and once he determines that my request for a brief interview is serious, he, too, proves capable of being serious. He and his group of three, it turns out, have coincidentally just come from the Tastee 29 in nearby Fairfax, one of the few diners around that serves beer (although as an employee there says, "We don't push beer, we sell it. We don't advertise it as you can see. You just have to know we have it").

Despite some ribbing from the other three, Marks has this to say about the popularity of the diner:

"It may not be the best way in the world to make money, I don't know, but there's always going to be people coming to a diner. I don't think you'll ever see these places disappear. Where else can you go out at night to have a good time and then come back to in the morning to have breakfast and read the paper?" fwkruler,12DINER DIRECTORY

The following is a sampling of diners in the Washington area:

BETHESDA TASTEE DINER -- 7731 Woodmont Ave., Bethesda, 652-3970. Since 1939, at present location since 1942. Open 24 hours. Owner: Gene Wilkes.

BOB AND EDITH'S DINER -- 2310 Columbia Pike, Arlington, 920-6103. Since 1969. Open 24 hours. Owners: Bob and Edith Bolton.

ENGLISH DINER -- 22nd Street and Philadelphia Avenue, Ocean City, Md., 301/289-7288. Since 1939. Open 6:30 a.m. to 2 p.m. Owner: Allen Bunting.

FOX DINER -- 20 South Street, Front Royal, Va., 703/635-3325. Since 1955. Open 4:30 a.m. to 7 p.m., Monday through Friday; 7 a.m. to 7 p.m. on Saturday; and 7 a.m. to 2 p.m. on Sunday. Owner: Loretta Wines.

FROST DINER -- 55 Broadview Avenue, Warrenton, Va., 703/347-3047. Since 1955. Hours vary. Owner: Robert L. Ward Sr.

LAUREL TASTEE DINER -- 118 Washington Boulevard, Laurel, 725-1503. Since 1951. Open 24 hours. Owner: Gene Wilkes.

NICK'S DINER -- 11199 Viers Mill Road, Wheaton, 933-5459. Since 1977, although the diner has been in existence "for more than 60 years," according to its owner. Open 5 a.m. to 9 p.m. Owner: Nick Markopoulos.

SILVER SPRING TASTEE DINER -- 8516 Georgia Avenue, Silver Spring, 589-8171. Since 1938. Open 24 hours. Owner: Bob Traynor.

TASTEE 29 DINER -- 10536 Lee Highway, Fairfax, 591-6720. Since 1942. Open 24 hours. Owner: Richard Millikin.

James Lilliefors, a frequent Weekend contributor, mostly wrote this story in the English Diner at Ocean City.