Who wrote the lyrics to West Side Story? Here's a clue: he shares his birthday with Andrew Lloyd Webber. Here's another: his 1971 musical is currently being revived at the Royal Festival Hall after a 15-year absence from the London stage. Stephen Sondheim is the man, of course, and Follies the show.

For theatre aficionados, this may seem a simple question, but the fact of the matter is, Stephen Sondheim is not the household name you might think. After all, Sondheim has yet to have a show celebrating a 21-year run on a par with Cats - a show that made his birthday twin Lloyd Webber's name almost synonymous with "smash hit."

Composer Comparisons

And yet Sondheim is credited as being one of the greatest musical writers of the 20th century. In addition to Follies and West Side Story, Sondheim's works include Gypsy (lyrics), Sunday in the Park with George, Sweeney Todd and Into the Woods. Productions of his shows have received a sizeable collection of Tonys, Oliviers and even a Pulitzer Prize. Two collections of serious literary and musical studies have recently been published alongside several weighty biographies - all exploring his life and work.

To his name, Lloyd Webber has a stack of multi-million pound hits, many of which became (almost) permanent fixtures in the West End and on Broadway. In London, Cats has only just closed after 21 years while The Phantom of the Opera is approaching its 16th birthday. Lloyd Webber's most recent venture, producing Bombay Dreams, was seen as an unusually risky step - but it's been packing them in at the Apollo Victoria and, only opened in June, has already announced a six-month extension to booking with a Broadway transfer and even a film version looking increasingly likely.

Comparing achievements on paper, you'd think you had two worthy composer competitors. Yet none of Sondheim's musicals have racked up anywhere near the same number of performances as Cats or had the financial success that comes with it. Why is this? What makes the likes of Into the Woods or Follies so different from Phantom or Starlight Express?

Early beginnings

Born in New York City on 22 March 1930, Stephen Sondheim moved with his mother to Pennsylvania after his parents' divorce. It was a significant move in every sense of the word because it was in Pennsylvania that Sondheim met the Hammerstein family and was taken under the wing of the great man himself, Oscar Hammerstein II. Under his mentor's guidance, Sondheim wrote the music and lyrics for several "practice musicals", and in 1957, was persuaded to take a job writing the lyrics for a new Leonard Bernstein musical called West Side Story.

The huge success of West Side Story established Sondheim as an extremely important lyricist, and that reputation cemented was with his follow-up, Gypsy. But Sondheim wasn't content with words alone. In 1962, A Funny Thing Happened on the Way to the Forum opened with his name on the billing as composer and librettist. It was another success, running for two years, and with his next major Broadway project, 1970's Company, Sondheim was confirmed as a groundbreaking force in musical theatre.

Andrew Lloyd Webber, meanwhile, began his career in his twenties, writing music alongside Tim Rice's lyrics. The pair's first musical, Joseph and the Amazing Technicolor Dreamcoat, was originally conceived for a school production and later developed into a full show. They followed it with 'rock opera' Jesus Christ Superstar and, in 1978, Evita, based on the Argentine first lady. Jesus and Evita became the most internationally successful musicals of the decade, dwarfing, even at this early stage, Sondheim's commercial achievements with Forum and Company.

Hummability & Happiness

A lack of 'hummability' is the most oft-mentioned criticisms about Sondheim's music. Many also protest that it's too intellectual or too difficult to listen to. Sondheim acknowledged such views in Merrily We Roll Along, in which a blundering producer grumbles about a new musical:

"There's not a tune you can hum
There's not a tune you go bum-bum-bum-di-dum
You need a tune you can bum-bum-bum-di-dum
Give me a melody!"

In fact, there are two kinds of hummable: songs that the audience know already or catchy tunes that are easy to pick out.

A canny marketer who knows the power of a single, many Lloyd Webber musicals benefit from songs people will have heard in the charts before they enter the theatre, for example, "Close Every Door" (Joseph), "Don't Cry for Me Argentina" (Evita), "Memory" (Cats) and "No Matter What" (Whistle Down the Wind). Sondheim's shows, on the other hand, are often more complete works, whose individual songs don't necessarily make sense taken out of context (two notable exceptions have been "Send in the Clowns", with which Judy Collins, Frank Sinatra and Barbara Streisand all had hits, and "Losing My Mind", recorded by Liza Minnelli).

As for catchy tunes, showstopper numbers are missing from Sondheim. Where Joseph has its title song and the "Go, Go, Go Joseph" chorus (complete with Sha, la, la's), Sondheim offers haunting melodies such as the beautiful "The Road You Didn't Take" from Follies. Like opera, the music often needs to be listened to a few times. Even those singing it find it a challenge. In her recent 20 Questions interview with Whatsonstage.com, Kathryn Evans, playing Sally in the current production of Follies, commented, "The music is difficult and you don't always expect where the melodies are going to go. But when you get it right, it's so rewarding."

Sondheim also chooses decidedly difficult subjects for his musicals: a serial-killing, meat pie-stuffing barber (Sweeney Todd), a trawl through the attempts made on the lives of American presidents (Assassins). And, as if that weren't enough, he's not averse to removing traditional concept of a story having a beginning, middle and end: Company, about bachelor Robert, is presented as a series of miscellaneous flashbacks while Merrily We Roll Along, centring on three friends, moves ceaselessly backwards in time. Even when Sondheim does stick to traditional story material - as with Into the Woods' fairy tales - he opts to subvert the "and they all lived happy ever after" with an uneasy "to be continued". In fact, that absence of the 'happy ever after' is a recurring theme in all his work.

No Chance of Escape

Out of all types of theatre, musicals are considered the most 'escapist entertainment' - you can sit back and enjoy without taking any of it too seriously. Or that's the popular perception, and for the more accessible Lloyd Webber, it by and large holds true.

Not with Sondheim. His musicals so closely mirror real life and real, powerful emotions - Follies' portrayal of middle age regrets and ravaged relationships, for instance - that there's no chance of escaping. And, though not always articulated, this may well be the primary reason that many potential fans are put off, despite the beauty of the scores: Sondheim's musicals are about the things most people go to musicals to get away from.

As a result, Sondheim is often regarded as "caviar to the general", with a legion of devoted connoisseurs rather jealously guarding the exclusivity of their interest. But with escapist feelgood fodder like Fame, We Will We Rock You and Grease bulking out the West End again this season, shows like Follies are vital to demonstrate what else musicals can be.

Follies continues at the Royal Festival Hall until 31 August 2002. Stephen Citron's book Sondheim & Lloyd Webber: The New Musical (Chatto & Windus, £25) also explores the similarities and differences between these two composers.