"Two and a Half Men" (search) is a success, and why not?

The CBS comedy is smartly cast, with Charlie Sheen (search) and Jon Cryer (search) as mismatched siblings, one a slick womanizer, the other a hapless nerd. Holland Taylor is their icy mother and promising young actor Angus T. Jones is the cuddly kid in the middle.

Co-creator and executive producer Chuck Lorre is a sitcom master who knows what's funny, with a resume that includes "Roseanne," "Grace Under Fire," "Cybill" and "Dharma & Greg."

So why should it be at all surprising that the show, building on its first-year ratings at 9:30 p.m. EDT on Mondays, is in the top 10 at the season's start?

Because while "Two and a Half Men" is getting belly laughs, TV comedy itself is gasping for air.

A decade ago, 11 of the top 20 shows were situation comedies, including No. 1 "Seinfeld" and top-five shows "Home Improvement" and "Grace Under Fire." But last season, only four sitcoms cracked Nielsen Media Research's top 20 and just one, "Friends," was in the top five.

And after two weeks of the new season, that pattern is repeating itself as crime and reality continue to dominate the ratings.

"We sort of blew the narrative," said Cryer, who plays an unhappily divorcing dad. "The story was 'The sitcom is dead,' and then, oops, under the radar we're the No. 2 show last summer."

They had to earn their ratings stripes, defying both viewing trends and the conventional wisdom that the traditional sitcom — based in a home or office and shot in the multi-camera format born last century with "I Love Lucy" — was over.

The CBS show is averaging 16.5 million viewers so far, compared to its 2003-04 average of 15.3 million. The numbers are expected to rise when ABC competitor "Monday Night Football" wraps for the year.

So much for conventional wisdom — which even writer-producer Lorre had bought into.

"After spending many years during four-camera comedy, I really thought the genre needed to be deconstructed. And I tried, and I failed," said Lorre, referring to a 2001 interactive comedy pilot, "Nathan's Choice."

"When we started writing this ('Two and a Half Men'), we were able to take a very traditional approach and I discovered, much to my delight, there was really nothing wrong with the genre.

"If you create characters you care about and relationships you care about, and you keep your standards as high as you possibly can for what you think is funny and interesting, then it's wonderful. ... It's not broke."

"Two and a Half Men" is closer to the divorced Lorre's own life than other shows he's handled, making him a very attentive steward of the Harper clan.

That includes Charlie (Sheen), a jingle writer for commercials living the good life in Malibu; sad-sack brother Alan (Cryer), a chiropractor tossed out by his wife and taken in by Charlie; and Jake, Alan's 11-year-old son, a weekend visitor with dad and uncle.

The boy's effect on irresponsible Charlie, Alan's adjustment to singlehood and the rocky relationship between the mismatched brothers is the heart of the show.

"The other thing we wanted to do was tell an honest story about divorce and custody," Lorre said, speaking for himself and co-creator/executive producer Lee Aronsohn. "We'd both been through it and this was a chance to do it."

The actors are protective of their characters. Sheen, who has traded in his wild playboy past for family life (he and his wife, actress Denise Richards, have a 6-month-old girl) insisted the fictional Charlie avoid sleaziness.

"I said, 'You can do stuff where I play a womanizer, play a bachelor, play a guy whose moral fiber might be a little questionable,'" Sheen told The Associated Press. "But I said just do it tastefully and don't beat it into the ground. Just do it intelligently."

Cryer jokes that he failed to see the similarities between him and the nerdy Alan: He thought it odd his friends insisted it was perfect for him, when he believes he's so much "cooler and sexier than this guy."

"I really wanted to believe I wasn't like him, and sadly I am. I'm a very repressed guy," said Cryer.

A veteran of several short-lived sitcoms ("Partners," "The Trouble with Normal"), Cryer had lost perspective on what made a show work.

"When you latch onto a great character, like I think Alan is, it's mostly a relief. You go, 'I know who it is,' and it just makes the way ahead clear," he said.

Sheen, who took over for Michael J. Fox on "Spin City" in the mid-1990s, already knew the pleasure of working on a successful sitcom. He learned a crucial lesson from its creator, Gary David Goldberg.

"We'd finish a (script) read-through on a Monday and I'd see him walking around, shaking his head, and I'd say, 'Gary, what's wrong?' And he'd say, 'We're completely out of our world.'"

A series has to stay true to what makes it distinctive, Sheen said — and Lorre concurs. He describes the "litmus test" that each "Two and a Half Men" episode must pass.

"Is there any other show on television that can tell this story? If so, let's not do it, it's generic. How is it specifically a show about our characters, one that only we can tell?"

The other immutable rule, according to Lorre: Know the difference between amusing and funny.

"And there is a difference. Funny makes people laugh. You honor that, and you can put on a show that has a chance of surviving."