Every once in a while a play arrives that detonates an entire genre. The "well-made" West End parlour room plays of Coward, Rattigan and the like that kept audiences amused in the 1940s and 50s, for instance, were blown away in 1956 by the arrival of John Osborne's Look Back in Anger, the drama that exchanged the parlour for the kitchen sink (and ironing board) with a new kind of documentary-like realism.

At last year's Hay Literary Festival, David Hare commented that Look Back in Anger, plus The Entertainer and Inadmissible Evidence that followed it, were "not important for what they are said to have removed from the English stage - good taste, irony, lame jokes, and rigidly chewed upper lips - but revolutionary for what everyone now forgets they put in their place: strong feeling." Another writer, Alan Sillitoe, has put the Osborne's influence even more succinctly: "Osborne didn't contribute to British theatre; he set off a landmine and blew most of it up." For Arnold Wesker, Osborne "opened the doors of theatres for all the succeeding generations of writers".

Reverberations

Osborne, Sillitoe and Wesker came to be known collectively, with others, as the "Angry Young Men" of British playwriting in the 1950s, just as a new breed in the 1990s came to be associated with the phrase 'in-yer-face' theatre, which, in critic Aleks Sierks' definition, is theatre that "shocks audiences by the extremism of its language and images; unsettles them by its emotional frankness, and disturbs them by its acute questioning of moral norms."

It's epitomised by plays like the late Sarah Kane's Blasted and Mark Ravenhill's Shopping and Fucking, which - like Osborne's seminal work - also both perhaps not coincidentally arrived at the Royal Court, in January 1995 and September 1996 respectively, and ushered in a new era in British drama whose reverberations are still being felt in this new century.

No wonder that, The Mousetrap apart, the West End is largely unrecognisable from the place it was 50 years ago or even 20 or 30. Theatre, being a living art, is constantly mutating, taking on new forms and discarding old ones. But what's also interesting is how quickly that process can take hold.

West End staples no more

The stage thriller, for instance, was once a West End staple, but has now been all but lost - largely due to the fact that Anthony Shaffer's Sleuth, premiered in 1970, intricately wrote them out of its existence with its Pirandello-like game-playing on the nature of truth and illusion that few could dare follow. By deliberating undermining its own genre - the British drawing room murder mystery - but giving it a contemporary spin, Sleuth ensured that there was nowhere left for it to go.

Likewise, Michael Frayn's Noises Off at once deliciously celebrates yet simultaneously skewers another repertory war-horse, the drawing room sex comedy, as it shows a production of one literally falling apart from behind-the-scenes. The 1982 play - celebrating its 21st birthday now with a return to the West End's Piccadilly Theatre in what is, appropriately, a touring version of the National Theatre's hit revival - has also fatally but hilariously undermined the very genre it's based on. As the play-within-the-play - called Nothing On - collides with the backstage farce of putting it on, what has been described as "a single collective nervous breakdown" occurs and you could never watch a 'serious' sex comedy again in quite the same light.

No wonder that they are now, the occasional entry by master farceur Ray Cooney excepted, mostly extinct. So, while the likes of No Sex Please - We're British and Run for Your Wife both ratcheted up record-breaking runs in the West End in the late 1970s and early 80s, it's now Noises Off that seems to refuse to go away. The Guardian's Michael Billington, reviewing the original production in 1982, named it "a pulverisingly funny play... which shows the precarious illusion of theatre reduced to total chaos." But it's a minutely calibrated kind of chaos that plots the confusion with meticulous planning.

Constant mutation

Often, however, the revolution isn't quite so dramatic: forms like melodrama, vaudeville, burlesque and music hall that dominated the theatre in the 19th and early 20th centuries have all had slower fade-outs from popular appeal and consciousness, while the musical - arguably the most popular of all forms of commercial theatre - seems to be in a constant state of mutation. Every now and again a defining moment occurs: as legendary American composer Richard Rodgers succinctly observed in his autobiography, "One night a show opens and suddenly there's a whole new concept".

Rodgers was himself responsible for one of these landmark moments with his and Oscar Hammerstein's hit Oklahoma! that, in 1943, ushered in the age of the 'integrated musical' where book, music, lyrics and dance all contributed towards the telling of a serious story.

Later, Stephen Sondheim and his collaborators were credited for developing the 'concept musical' that revolves around a central idea rather than a linear story. More recent 'innovations', like the through-sung, spectacle musicals of the 1980s that were a big feature of the British theatrical landscape and were exported internationally, are arguably not so much new as throwbacks to an earlier age of the musical's origins in opera and operetta.

Whether innovation or merely variation on an old theme, theatrical form resists easy categorisation - and even as new trails are blazed to the future, old forms sometimes need to be put gracefully (and sometimes disgracefully) to rest. There is no finer epitaph to the sex comedy than the sheer pleasures of Noises Off. As Frank Rich, former chief critic of the New York Times, puts it, the play "was, is, perhaps always will be funniest play written in my lifetime".