Meg Ryan and Billy Crystal prove that men and women can’t be friends in "When Harry Met Sally..." (CASTLE ROCK ENTERTAINMENT)

Since its release in 1989, nearly every romantic-comedy has tried to measure up to “When Harry Met Sally...,” the valentine to cross-gender friendship written by Nora Ephron, who died Tuesday at the age of 71.

Every time we plunk down $10 to $14 and walk into a stadium-seating auditorium to watch one of those rom-coms, we hope that this time, someone succeeded. But 9.89 times out of 10, they don’t. That’s partly because Hollywood executives seem to think that what we want from contemporary love stories is predictable, insulting pablum that stars interchangeably recognizable female stars as the gorgeous yet shockingly clumsy protagonist d’jour.

But most of these movies also can’t meet or exceed the “Harry Met Sally...” bar for another reason: because Ephron’s screenplay is basically perfect.

Obviously Rob Reiner’s direction and contributions to the script also were key to the film’s effortless charms, as were its performances, particularly by leads Billy Crystal and Meg Ryan. I never in a million years would have pictured those two as a couple if this film did not exist. But now, despite Ryan’s previous relationships and Crystal’s long, happy marriage to his wife Janice, I will forever hang on to the image of Crystal and Ryan — Harry and Sally — together, in love, with their frizzy, late ‘80s hair and their mutual agreement that the chocolate sauce for their wedding cake needed to be served on the side.

“On the side” is a very big thing in this movie. So is high-maintenance. Did anyone use the term high-maintenance on a regular basis until this movie came out? No, no one did. Then everyone started to categorize everyone else using that as a metric — “An L.M. Definitely.” — so that by the time the early ‘90s rolled around, you pretty much knew where you stood on the Nora Ephron maintenance divide. (The fact that I remember this so clearly suggests that I am high maintenance. But really, I’m the worst kind, because I think I’m low-maintenance.)

View Photo Gallery: Ephron, who emerged over the past four decades as one of America’s warmest and most acute chroniclers of contemporary culture and society, has died.

Ephron put words in these characters’ mouths that, at least in the late 1980s, we didn’t hear in movies very often. As the Post’s Adam Bernstein noted in his obituary of Ephron, she can’t take credit for the famous “I’ll have what she’s having line” (that was all Crystal’s idea). But the notion of a scene in which Sally breaks the news to Harry that many women fake orgasms — a moment that would be recreated a few years later in an episode of “Seinfeld” — is something for which she was partially responsible.

At the time, it was a revelation that, during my first screening of the movie in the summer of ‘89, sent every patron at the packed Cineplex Odeon White Flint in Rockville, Md., into hysterics for multiple minutes.

Yet while it’s the most well-known scene in the movie, it’s not my favorite. To me, that scene is the big top-40 hit on an album that’s nothing but wall-to-wall awesome deep tracks.

Even today, if I’m in a Sharper Image or a Brookstone store, I always feel an urge to pick up a microphone and sing “Surrey with the Fringe on Top.”

At bookstores, I always check to see if anyone is staring at me in Personal Growth.

I have never played a game of Pictionary without randomly screaming, “Baby fish mouth! Baby fish mouth!”

Ephron wrote a movie that, like very few rom-coms, now or ever, wastes no moments and delivers dialogue that lands every time with purpose and zing. The conversations between Harry and Sally are like a tennis match that’s all volley, nothing but over-the-net.

Did she borrow from previous works? Sure. The notion of a man and woman who initially hate each other then develop more tender feelings was ripped straight from “It Happened One Night” and “Bringing Up Baby,” to name just two rom-com classics.

And as I noted in this DVD review, “When Harry Met Sally...” also cribs more than a few elements from “Annie Hall,” Woody Allen’s 1977 portrait of neurotic New Yorkers in an on-again, off-again relationship. Ryan’s Sally even dresses a little like Keaton’s la-di-da Annie at certain moments.

What Ephron did was take those all those familiar rom-com elements and give them, simultaneously, a sharper, more modern edge than what we witnessed in the screwball comedies, and a mushier heart than the one at the center of “Annie Hall.” But it all felt natural and uncontrived because it was written very clearly with her voice. It just worked.

Thanks to its success at the box office (it made more than $90 million), “When Harry Met Sally...” set the stage for much of the whip-smart observational entertainment that would follow in years to come, like the aforementioned “Seinfeld,” “Sex and the City,” all those friends-with-benefit movies, every film that inserts vignettes in which people or couples talk to the screen (looking at you, “He’s Just Not That Into You”) and any post-’80s romance that’s set in New York and/or attempts to enchant us with a climactic New Year’s Eve scene.

(Seriously, why would anyone watch “New Year’s Eve” when they could get everything they need in that department from the final five minutes of “When Harry Met Sally...”?)

During those final five minutes, Harry makes a speech in which he questions the meaning of the song “Auld Lang Syne.”

“I mean, should old acquaintance be forgot. Does that mean we should forget old acquaintances, or does it mean if we do happen to forget them we should remember them, which is not possible because we already forgot them? ”

“Maybe you’re just supposed to remember that you forgot them, or something,” Sally guesses. “Anyway, it’s about old friends.”

That’s what “When Harry Met Sally...” is to the many who still love it after the 873rd viewing (number approximate). It’s an old friend, one we’re very thankful that Nora Ephron so graciously introduced us to.