Tracy and Hepburn. Bogey and Bacall. Fred and Ginger. Abbott and Costello.

The most memorable movie tandems possess that most elusive and ineffable of interpersonal traits -- chemistry.

"Chemistry can be either spontaneous or formulaic," says Toby Miller, a film professor at New York University. "We've all experienced that spark of understanding and attraction that happens when we first meet someone. And then there's the chemistry that comes with working with another person for a long period of time, when a nod or a wink is as good as a stage direction." "Sweet November," a romance that opened Friday, reunites the pretty stars of "Devil's Advocate," Charlize Theron and Keanu Reeves. Perhaps due to Theron's remarkable charisma and statuesque physique, it's difficult to find a co-star who's her match onscreen. Reeves, though lacking key acting skills, nonetheless comes close to fitting that bill. If nothing else, "Sweet November" promises the possibility of another silver-screen coupling sprinkled with the electrical fairy dust of chemistry.

Says Reeves about the pair's history: "Charlize is a better actress now. She's more of a good thing. She's beautiful and funny and strong. She's a woman." "They're perfect casting," says "Sweet November" co-star Jason Isaacs. "Keanu and Charlize are two opposites who like and respect each other, but on paper you wouldn't think their relationship could work because they're about as different as two people can get."

Indeed, the tension of opposites is one of the secrets of movie chemistry. As with such comedy teams as Dean Martin and Jerry Lewis, Bing Crosby and Bob Hope, Stan Laurel and Oliver Hardy, and Bud Abbott and Lou Costello, all of which comprised a relatively suave straight man and a lovable goofball, many successful pairs have thrived on contrast.

Spencer Tracy's hail-fellow-well-met gruffness complemented Katharine Hepburn's aristocratic brittleness. Humphrey Bogart's craggy, middle-aged face and non-filter voice were offset by Lauren Bacall's bursting, succulent-lipped youth. Jack Lemmon's Felix butted heads with Walter Matthau's Oscar. Mel Gibson's flamboyant cowboy was neutralized by Danny Glover's cautious family man and Woody Allen's cerebral whining was, sometimes, calmed by the upbeat shiksaness of Mia Farrow and Diane Keaton.

Theron and Reeves may not make anyone forget Tracy and Hepburn, but their movies do pair a bubbly Charlize with the stiff Keanu, making them possible cinematic foils for future exploitation by the right filmmaker.

"In recent years, there haven't been many longstanding movie pairs," Miller says. "When you have big stars who share a connection, it's hard to get them back together in another movie, especially when one of the stars gets bigger." Miller points to examples like "Hannibal," with its inability to re-team Anthony Hopkins and Jodie Foster, and 'Speed 2," in which Jason Patric was called in to replace Reeves, opposite Sandra Bullock.

The promising pairing of Richard Gere and Julia Roberts made only two pictures together ("Pretty Woman" and "Runaway Bride"), and Tom Hanks and Meg Ryan have teamed for three ("Sleepless in Seattle," "Joe vs. the Volcano," and "You've Got Mail").

In another era, Tracy and Hepburn shared billing in 10 movies, and Bogey and Bacall in five. (Surprisingly, Rock Hudson joined Doris Day on the marquee for only three flicks, "Pillow Talk," "Lover Come Back," and "Send Me No Flowers.")

Some of these movie matings have appeal because they're cute squared. And while being cute and cuddly is often enough to launch a co-dependent career, it sometimes helps to sing, as with Mickey Rooney and Judy Garland and Nelson Eddy and Jeanette MacDonald, or dance, as in the case of Fred Astaire and Ginger Rogers, or Astaire and Cyd Charisse, or even Astaire and Gene Kelly.

Volatility is another binding factor for screen couples. Offscreen gossip helps fuel onscreen pyrotechnics -- the chemistry set always threatening to blow.

Fans of Richard Burton and Elizabeth Taylor were forever waiting for the next scandal, the next knock-down-drag-out, the next act of headline-worthy self-destruction. The same can be said, though slightly less sensationally, for the pairings of Corey Haim and Corey Feldman, Burt Reynolds and Sally Field, Roadrunner and Wile E. Coyote, and the aforementioned Reeves and his first multiple-movie buddy, River Phoenix.

Every era has had its dominant screen team. The 1920s boasted silent movie lovers John Gilbert and Greta Garbo. William Powell and Myrna Loy starred in 15 films together; most of them, with their wire-haired terrier, Asta, were the sophisticated "Thin Man" comedies of the Thirties. The 1940s were ruled, on the comedy side, by Hope and Crosby and, on the boy-girl side, by Tracy and Hepburn.

Any deep discussion of Fifties culture is incomplete without an analysis of what made Rock Hudson and Doris Day so popular, even into the Sixties, when Liz and Dick boozed and brawled their way through the decade.

Circa 1970, Paul Newman and Robert Redford, though they made only two movies together, were iconic in "Butch Cassidy and the Sundance Kid" and "The Sting." "If there were a good-enough script, I'd do a movie with Bob again tomorrow," Newman has said.

The Eighties gave birth to the "Lethal Weapon" interracial buddying of Gibson and Glover, who were a slicker version of the "48 Hrs." coupling of Nick Nolte and Eddie Murphy, who were, in turn, a twist on the Gene Wilder-Richard Pryor marriage.

The top movie couple of the 1990s was Kevin Costner and his ego, co-stars of "Dances With Wolves," "Waterworld," and "The Postman" -- an incredible run for the dynamic duo -- though the "Thelma & Louise" team of Susan Sarandon and Geena Davis made history as the first female buddies.

Who will be the top Tinseltown team of the '00s?

Chances are, it won't be Tom Cruise and Nicole Kidman, who apparently generated as much heat in their real-life marriage as they did onscreen in "Eyes Wide Shut."

And chances are, according to Miller, the new millennium mates won't become famous doing the usual thing -- romantic comedies.

"I'd love to see cross-gender teams in action-adventure movies," Miller says. "Maybe there will be more 'Charlie's Angels' movies. The macho action-adventure seems to have run its course."