"What are you drinking?" Jeff Ellis asked his twin brother, Bruce, at the reception last night celebrating a book about twins. He already knew the answer, of course.

"Gin and tonic," said Bruce. "We drink bourbon if the weather's under 70 degrees. When it gets above 70, we switch to gin and tonic."

"yeah," said Jeff, "and then we trade wives.

This is an old twin joke. The Ellis brothers laughed pretty hard anyway.

"I used to go out with Bruce's girlfriends, and they were so dumb they never knew the difference," said Jeff.

"I don't think that's too damn funny, Jeff," said Bruce. Then they laughed again.

The Ellis brothers are vice presidents of Rigewell's Caterers, so they were the twins who brought the food. They also brought their twin nieces, Megan and Shannon Delaney, age 5, to the party in the Rayburn Building. They weren't alone.

There were twins from twins associations, and there were associations of the parents of twins, and there were even a few grandparents of twins. And then there were the twins of honor, Kathryn McLaughlin Abbe and Frances McLaughlin Gill, who've written a book about the whole thing.

"We wrote 'Twins on Twins' because we found they just weren't any books on twins, except medical texts and freaky books, you know, about twins who kill each other or somthing like that," said Abbe. She and her sister are professional photographers. They spent two years on the book. Last night they were wearing identical blue skirt and sweater sets. "We don't usually do that," said Gill. "It's for the book. Donald Keith said we ought to."

And Keith should know. He's the president and founder fo the Virginia-base Center for the Study of Multiple Gestation. His twin brother, Louis, an obstertrician who has delivered his share of twins, was at the party, too. Rep. Adam Benjamin Jr. (D-Ind.) was another host, but a last-minute budget markup kept him away. His twin didn't show either.

"There are three different views of twins," said Donald Keith. "There's the media's, which is that we are unusual. Then there is society's which compares us. That's the worst."

And the third?

"Our own," said Keith. "Remember Ford turns out identical cars, but nature doesn't turn out identical people."

The McLaughlins' parents took pains to give them separate identities, didn't even saddle them with similar-sounding names. So what did Kathryn do? "I made up a new name for myself when I was very young. I made them call me Fuffie."

Nearby, Geoffrey and Stephen Koch were perched side by side on banquet chairs. Geoffrey wore a blue jacket, Stephen, light brown. Stephen's face looks a little fuller. Maybe. He keeps his tie clamped in place with a coin tiepin. They both have big smiles and new big teeth.

"We're 12 going on 13," said Geoffrey. "He builds model tanks and more warlike things. I build more trains."

"Right," said Stephen, nodding his head.

Geoffrey does most of the talking.

"I don't mind it," said Stephen.

"I have thought what he's thinking sometimes," said Geoffrey.

"One time we must have been thinking of the same thing," said Stephen.

"Because we said the same thing at the same time," finished Geoffrey. They looked at each other and smiled contently, like two very old friends.

"Oh, I wish I'd found them in time for the book!" said Frances McLaughlin Gill.

Or was it her sister?