"It's basically making love with your clothes on."

That's Chubby Checker's succinct explanation for the enduring popularity of the Twist, the dance that helped launch his career.

On Friday, Checker, 68, with clothes on (jeans suit, open shirt), delivered 45 minutes of straight-ahead '60s rock-and-roll to hundreds of delighted twisters in Dilworth Plaza, in a free noontime concert billed as 50 Years of Twistory.

Fifty years ago Friday, Checker's version of "The Twist" was released by the Philly label Cameo-Parkway. He performed it that night at the Rainbow Club in Wildwood, "the birthplace of the Twist."

Soon he was on American Bandstand and seemingly all over TV. "The Twist" hit No. 1 in September 1960 and again in January 1962. It's still the only tune to hit the top twice on the Billboard Hot 100, and in 2008 it was certified as the most popular tune in the list's history.

Some historians think the Twist led to the decline of touching in American popular dance. Speaking by phone this week from his home in Conestoga, Checker said: "The Twist is about dancing by yourself on the dance floor with someone else who is watching you do it."

Longtime radio guy Jerry Blavat, "the Geator with the Heater," who emceed Friday's show, concurred: "To do the jitterbug, the swing, and the Latin dances, you had to know complicated steps, but with the Twist, everyone could do it. It was dancing apart, but the way you felt like dancing."

The real theme of the show was that Philadelphia was ground zero for this explosion in pop history. On Friday, Blavat told the crowd, "You are the real stars," and in that spirit Checker, with his band the Wildcats and dancers the Checkerettes in support, hauled groups of guys and groups of gals onstage to dance with him.

He also displayed his prowess at the Pony, the Fly, and the Hucklebuck. ("Back in the '60s, you couldn't do the Hucklebuck because it was too nasty. Now it's 2010, and everything's nasty.") Folks were also spotted doing the Limbo. (Checker had a hit with his version of "Limbo Rock.")

Born Ernest Evans in Spring Gully, S.C., Checker came with his family to Philly at age 8, and little more than a decade later, he made "The Twist." All the musicians on the record were from Philadelphia, as was producer Dave Appell.

American Bandstand, Dick Clark's hugely influential pop and dance show, was then filmed at the WFIL studios at 46th and Market Street. "The real key" to "The Twist," Blavat said, "was Dick Clark. 'The Twist' was originally by Hank Ballard, a B-side. Dick Clark heard the Hank Ballard song, called up Cameo-Parkway president Bernie Lowe and said, 'I think if you get Chubby to do this song, Chubby and the kids on Bandstand will develop the dance.' And that's what happened."

Little development was needed. Said Checker: "The Philly kids came up with the dance, and when Chubby Checker danced 'The Twist' on American Bandstand, dancing changed in America."

In fact, "the Philly kids" - some of whom may well have been among the dancers Friday - had been doing dances called "the twist" or "the grind" for years. Blavat recalled outdoor neighborhood record hops in South Philly in the late 1950s in which young people scandalized the neighbors with the Twist.

They probably didn't know it, but forms of the dance - dancers apart, gyrating hips while facing each other - had been around for centuries and can be traced to Africa.

"That was the roots of the Twist, and also the Pony [Checker's tune "Pony Time" also hit No. 1], the Fly, the Shake, and the Hucklebuck," Checker said. "I first saw the Hucklebuck in 1949, but it didn't go anywhere. It wasn't until the Twist that all these other dances broke. What's so exciting about it is, it's a striptease with your clothes on, a lure, it's sexual."

The popularity of singer, tune, and dance was as clear as the hot sun in Dilworth Plaza. The median age of attendees seemed to be 40 and up.

"When I was young, we twisted the night away," said Debbie Lee, 55, of Philadelphia. "I remember seeing him on TV, playing at the Steel Pier in Atlantic City." Brooklynite Sandra McCollum, 55, saw him perform at Madison Square Garden, while John Rosado, 51, of Philadelphia, said, "I saw Chubby play a show at a club called Heaven outside of Levittown."

Younger twisters were there, too. Rebecca Trenholme, 18, of Northeast Philadelphia, said, "I learned it from my mom. All my friends know how to do it, and lots of times we'll do a twist at school dances." Victoria Anderson, 10, said, "Everybody knows how to do it because it's so easy."

Although a stab at a Guinness Book of World Records citation for the most twisters in one place did not materialize (the official observer never made it), Chubby Checker on Friday helped celebrate himself, his city, and, first and last, the people who did the dances.

Contact John Timpane at 215-854-4406, jt@phillynews.com, or twitter.com/jtimpane.