THEY HAVE BEEN comfortable and familiar with royalty, these two assured men sitting so casually in the empty restaurant. It's that gray time between meals, and one busboy noisily separates the silverware while another stacks metal trays. It sounds as if Buddy Rich is loose in the pantry.

Joe Williams and Billy Eckstine, two great American singers, sit back, safely inside the history they're talking about. The grand names bring smiles, memories of close encounters, troubles shared, joys provoked.

The Duke.

Prez.

The Count and the Earl.

Lady Day.

The Vibes President.

They are America's royalty, gentle jazz giants named Ellington, Young, Basie, Hines, Holiday and Hampton. Some are gone, but the royal jewels live on in songs left behind, in countless musicians inspired and taught and encouraged.

Williams and Eckstine, who have both lived in Las Vegas for years, are covering the Georgetown waterfront this week, working a block apart (at Blues Alley and Charlie's, respectively). They've known each other more than 40 years, but a life in music means that paths seldom cross often enough. This week, with a shared hotel as a base, is a good time for tale-swapping and reminiscing.

Eckstine, still the sharp dresser at 69, is also still handsome as a fox. When his popularity became widespread in the late '40s with songs like "I Apologize," "My Foolish Heart" and "Fool That I Am," Time magazine dubbed him "the Sepia Sinatra." By then, Eckstine had already been a star attraction with the Earl Hines band and had led his own big band, a legendary unit that included Charlie Parker, Dizzy Gillespie, Dexter Gordon, Art Blakey, Miles Davis and Sarah Vaughn.

"I came to Chicago in '38 and met Joe about '40," Eckstine says in the softly mellifluous tones that have sent so many hearts fluttering. In his silvery blue jogging suit, he looks to be in his early fifties and there is still a sensual ease to his manner.

"That's right. I heard him sing, 'Souuuuuuth of the border, down Mexico way . . .' " croons Williams in the Eckstine manner. Williams is a dynamic singer who came out of the Count Basie band to establish his own strong solo career. At 65, he is tall and imposing with a built-in toughness. It's not hard to understand why Cannonball Adderley used him as John Henry, the Steel Drivin' Man, in his folk-opera "Big Man."

Both singers recalled the early '40s, when jazz clubs dotted most big cities and when there was round-the-clock work for those with enough endurance.

"Every little club in Chicago had a breakfast show every morning," Williams says, sipping an afternoon coffee with an ice-water chaser. "When the sun came up, we were always performing somewhere. We'd meet at Young's Drugstore, the corner of 55th Street and Michigan. All the musicians would come there in the morning when they'd get off their gigs. They'd stop in, have a sandwich, talk to people. It was a regular hangout."

"That's right . . ."

"We'd have malted milk and sandwiches. Do you remember apple pie with the cheese on top?"

"Still a good memory," says Eckstine, licking his chops.

"They would take American cheese, put it on top of the apple pie and stick it under the broiler . . ."

"It was delicious . . ."

"Oh man, then you'd get a scoop of vanilla ice cream . . ."

There were hard times, too, particularly on the road in the '30s and '40s when some Americans still defined Duke Ellington's vanguard compositions as "jungle music."

"I always thought it was the most sophisticated asphalt jungle I ever heard," Williams says.

The worst part for many musicians was touring the South, where lodging and food for "coloreds" were often impossible to come by.

"The longest time was nine days on the road without a room in a hotel," Eckstine recalls, cigarette smoke curling around his shoulders like a gathering storm, "and it wasn't because of money. We just couldn't get rooms. We stayed in the bus, even though the guys had plenty of money in their pockets. We'd have to get our food from a grocery store, or have the white bus driver go to a carryout and bring food out to us. We couldn't get out of the bus."

"It was rough, but it was also some of the best schooling I ever had," Eckstine adds. "I never thought the South would be what it is today. I remember at one hotel in New Orleans, after Earl Hines and I wrote 'Jelly Jelly,' we went up to a radio program and had to ride up in the freight elevator. Now we headline there."

And attention is finally being paid to the impact of artists like Ellington and Basie on both black and white culture.

"Duke was always an innovator," says Eckstine. "He was always the Duke, beautiful, elegant at all times. For young black guys coming up, it made them poke their chests out. It was something that had to be an inspiration to you. Everybody wasn't shufflin' out of the stable and things . . ."

"(Jimmy) Lunceford's band was another one," Williams adds. "They were always elegant."

"And Earl was very elegant. There were certain ones that you always felt good about."

"Ellington said, 'We always try to present our artists with dignity.' "

"That's the way he was. He was always the Duke."

Williams and Eckstine each have a trunkload of tales involving the legends of jazz who were, to them, friends, comrades in swing.

"I was in Chicago once, had a strep throat and couldn't work," Eckstine says, "so they put me in the hospital. I get this call from Little Rock, Arkansas, and it's Louis Armstrong. 'Hello, B?' (Eckstine mimics the classic growl.) Right away you knew who it was. 'Pops?' He said, 'I heard you got something wrong wid' your pipes. I'm going to send you something I been using . . . and I been clear for years.' He's going to clear my throat!"

If these two singers have their own wealth of memories, they've also provided them for their fans.

Says Williams: "When a man is holding a woman in his arms and something you sing causes them to hold each other a little closer, feel love for each other, feel their desire for each other and make a decision about how they're going to live the rest of their lives because of it . . . then they'll always look at you warmly together."

Williams' eyes seem ready to mist, but a twinkle intrudes. "Then there are the times when just the girl looks at you, you see, that happens, too."

A little furrow breaks above the twinkle. "Then there's times when just the man looks at you . . . usually not too friendly."

But most of the memories are friendly and it doesn't take long to realize these two gentlemen have had a ball with their lives. The gray that is somewhat more than a fleck in their hair has been earned with grace. And art.