When Harry Met Sally, still sharp, funny and touching after all these years, occupies an ambiguous place in the history of the romantic comedy. Nora Ephron's spiky script and Rob Reiner's fluid direction helped place a fading genre right back in the mainstream.

Meg Ryan became the 1990s own Doris Day and went on to speak Ephron's lines in hits such as Sleepless in Seattle and You've Got Mail. Sadly, the pale copyists took over and the rom-com was transformed into the patronising chick flick. That bubble duly burst and the genre is now about as saleable as the Mongolian Yurt drama.

Of course we shouldn't blame When Harry Met Sally. Ephron took tropes from the fast-talking 1940s and worked them deliciously into the unsettled sexual politics of the 1980s. Billy Crystal plays the archetypal heel who will need to change before he is worthy of his less frivolous female friend.

“Men and women can’t be friends because the sex part always gets in the way,” Harry tells Sally when they meet shortly after his graduation.

The film is uncertain as to its answer to this question. As you should by now be aware, they eventually become friends and eventually have sex. So Harry is wrong? Well, it wouldn’t be a rom-com if that relationship didn’t end up as a properly romantic one. Maybe he was on to something.

When Harry Met Sally doesn't take place in the real world. Like the comedies of Ernst Lubitsch and Woody Allen (both unavoidable influences), the film allows even its stupidest characters to say the funniest things.

That adds to an explanation of its apparent timelessness. Ryan and Crystal spark off each other vibrantly. The New York locations still have a glamorous untidiness that was soon to be swept away.

Oh, and that’s Rob’s mom delivering the famous punch line in Katz’s diner. You knew that.