Net users are getting a chance to enjoy some of Mozart's most rarely performed compositions.
The diary holds details of 145 of Mozart's compositions
A digital version of Mozart's musical diary is being put online by the British Library to help celebrate 250 years since the composer's birth.
The digitised diary lets people click on and hear music from the opening bars of many of the works it mentions.
One featured composition is "Little March in D" that, the library says, has almost never been performed.
From 12 January visitors to the British Library site will be able to browse a hi-tech version of Mozart's Verzeichnis aller meiner Werke (Catalogue of all my Works).
Mozart filled in pages in the Catalogue between February 1784 and his death in December 1791. In total it contains details of 145 of the composer's works.
On the left hand side of each double-page spread Mozart wrote about five compositions and entered the date each was finished; the title and which instruments should be used to play it. Often also included were the names of singers who performed it, where it was composed and who commissioned the work.
On the right-hand side of each double page was written the opening bars of the work. For its version the British Library recruited members of the Royal College of Music to perform the introductory bars so visitors can hear what each featured composition sounds like.
"By turning the pages of this fascinating manuscript online, users will be able to follow the path of Mozart's life in his final years through the entries in his own 'musical diary' and marvel at the outpouring of sublime music produced in such a short space of time," said Dr Rupert Ridgewell, British Library music curator.
Users can zoom in or click on the notes to hear the music
Mozart wrote entries for everything he composed which means that the Catalogue features the opening music from many lost works such as "Little March in D".
The online version features 30 pages and 75 musical introductions to some of Mozart's most widely-known works such as the Marriage of Figaro, Eine kleine Nachtmusik and The Magic Flute.
The range of works in the Catalogue shows Mozart's versatility as it includes entries for operas, piano music, chamber music, concertos, cantatas and vocal canons.
The work has been put online as part of an exhibition called "Mozart's Musical Diary" which runs until 10 April in the Library's John Ritblat Gallery.
In total 15 books and manuscripts now form the British Library's Turning the Pages collection. Works on show include The Luttrell Psalter, The Lindisfarne Gospels and Lewis Caroll's Alice's Adventures Under Ground