Ben and Jonathan Finn were only looking for a shortcut to a music O-level exercise when they set out on the route to becoming millionaires. Their task was to take a melody line and write a harmony for it in the style of JS Bach. They decided to write a computer program to do the task. These attempts, and other computerised musical experiments, were to sow the seeds of a highly profitable software package.

Sibelius arrived in April 1993. Within a year it was being praised for its speed and simplicity. John McCabe, who used it to compose his ballet Edward II, said: "When I saw Sibelius 7 I thought, 'This is the one for me.' " Giles Swayne, the contemporary composer famous for Cry, among other works, describes it as "very, very addictive". The program has 15,000 users and the twins, who are 26, are well on the way to their first fortunes.

Accurate score-writing is as complex as it is arcane. From seemingly simple problems, such as spacing notes correctly and getting their tail lengths right, to concerns such as complex rhythmical alignments in large orchestral scores, the rules can prove to be both difficult and laborious. There were other notation programs, such as Score and Finale, before Sibelius. Composers could write music, on a computer keyboard or a simulated piano keyboard, and the program would display the score. But Sibelius's advantage was its simplicity.

"Laying out a page of an orchestral score by hand can take 15 minutes without putting any notes in," says Mr McCabe. "With this program, it doesn't take any time."

Adam Gorb, a modern composer and a reformed technophobe, is happy to demonstrate the speed at which he can create a score. Using a three-button mouse attached to an Acorn Risc PC, he can select the note pitch with a cursor, then adjust the length by pressing one of the function keys. As the score becomes more elaborate, further selections can be made from the many and detailed menus. And for composers who get their staves twisted, Sibelius Software runs a hotline.

Sibelius 7 has been incorporated into the composition curriculum at the Royal Academy of Music. Its playback facility means that students can listen to a synthesised version of their music.

The sophistication of the program can, however, have disadvantages. Mr Gorb, who also teaches at the Junior Royal Academy, comments that "certain students can believe that their works are a lot better than they really are, because the computerised presentation is over-flattering".

The key question is whether the use of a computer affects the work itself. Mr McCabe denies that the program inspires him, but admits that "it makes me enjoy the work more". He refuses to use the playback facility, though, because he believes he should have to imagine what the music sounds like. Mr Swayne says that one of his recent pieces for countertenor and organ is better than it would have been if written on manuscript paper. He says that having a piece on computer makes it "more like a sculpture. You have an object which you can shape and adapt".

Because Sibelius 7 can play music back, it can give technically high- standard performances. Its only public appearance has been at the Royal Academy of Music in March, when it played two pieces by Ligeti. The computer was linked to motors that depressed the piano keys. The performance delighted Ligeti, who is renowned for composing works beyond most human capabilities. The brothers are looking at ways of improving its expressive abilities. One day it might be able to give a convincing interpretation of Chopin.

Mr Swayne and Mr McCabe both respond favourably to the idea of composing music that no human could play. "I think that would be a perfectly viable means of artistic expression," Mr McCabe says.