One of the enduring mysteries in show business is how Bob Newhart and Don Rickles have been famously best friends for most of their professional lives.

Rickles, the in-your-face New York insult comic who bares the id barking behind every social exchange (to a woman in his audience wearing a mink stole, “You’re either a Jew or a beaver in heat”). Newhart, the mild-mannered Midwesterner forever in search of the right euphemistic word to blunt an uncomfortable truism (“Uh, Don and I are best friends, which shows how hard it is for me to make friends”).

Beyond the obvious — in this case a shared knowledge of the rewards and strains of life as an entertainer — friendship is impossible to define, particularly when one doesn’t need what the other has.

“We’re apples and oranges,” Rickles says. “I’m a Jew, he’s a Catholic. He’s low-key, I’m a yeller. But we share the same values, marriage, children and grandchildren. We’ve traveled all over the world together with our wives, and in all that time we’ve never had a serious argument. So many things happen, there’s so much sadness. But we can laugh together.”

The acid test came the night they met. Their wives, Barbara and Ginny, were best friends. Barbara invited the Newharts to catch Rickles’ lounge act at the Sahara. They sat in the front row, where Rickles spotted them.

“Ladies and gentlemen,” he said, “here’s the stammering comedian from Chicago and his hooker from Bayonne.”

The Newharts laughed and a mutual admiration was born.

“I don’t know how to describe it,” Newhart says. “When we travel, we don’t perform. We just enjoy ourselves. He’s nothing like his act. But when he’s on, I say, ‘How does he get away with that?’ One night, there was a guy sitting in the front row who had the world’s worst toupee. It looked like his wife threw it at him and it stuck. Don walked up and down the stage through his entire set, never saying a word. ‘How can he let this go?’ I thought.

“Finally, he looked at the guy and said, ‘No one will ever notice.’ ”

Newhart laughs, still disarmed at the memory. Maybe that’s a clue: laughter and friendship have no expiration date.