Richard Wagner was one of the greatest and most influential figures of the 19th century - not only as a composer of astounding originality, but also as a philosopher, poet and conductor. From an early age he was profoundly affected by literature, absorbing subject-matter from history and legend that later found its way into his own librettos on which his operas are based. Notorious in later life for his involvement in radical politics, his extravagant lifestyle and huge debts, and for his amorous entanglements with other men's wives, he was nevertheless a true visionary, a man with the sheer force of personality to realise his impossibly grandiose creations - massive music dramas which sought to fuse music and words in a new, indivisible union to create 'the music of the future'.

Most creative artists identify with their own heroes. Wagner may have seen himself as Wotan - the all-powerful, but also the fallible Wanderer - but when he abandoned work on the Ring cycle to write Tristan und Isolde, he certainly identified closely with that opera's tragic hero-lover. Based on an iconic legend of passion, betrayal and redemption, Tristan und Isolde was written under the influence of Wagner's own passion for a married woman, with its secret joy and attendant guilt.

During the early 1850s Wagner came close to despair. His marriage was in trouble, work on The Ring laborious and financially unsupported, and his health and spirits poor. Then, in 1852, he met Mathilde Wesendonk, the wife of a wealthy businessman. While Mathilde's husband Otto generously lent money to the struggling composer, Wagner¹s friendship with Mathilde ripened into a clandestine affair (although there is some doubt as to whether it was ever physically consummated). Under its influence, he conceived the idea of Tristan, writing to Liszt in 1854: 'As I have never in my life tasted the true joy of love, I will raise a monument to this loveliest of all dreams, in which from first to last this love shall for once be satisfied utterly. I have planned out in my mind a Tristan and Isolde, the simplest yet most full-blooded musical conception, then I will wrap myself in the black banner which waves over its close and . . . die . . .'

It took nearly another six years for Tristan und Isolde to reach the stage. On 10 June 1865 the opera was premièred at the Munich Hoftheater, conducted by Hans von Bülow (whose wife Cosima subsequently began an affair with Wagner, whom she finally married in August 1870). This turbulently emotional score - conceived under the influence of erotic passion - relates the tale of the illicit love of the knight Tristan and the Irish princess Isolde, the unwilling bride of King Marke of Cornwall - a love that may ultimately be consummated only in death. From the opening notes of the remarkable Prelude, it was clear that Wagner had finally left behind the world of conventional German Romantic opera to enter a metaphysical realm. The Prelude opens with a rising sixth, which then drops haltingly on to a spine-tingling diminished chord of superimposed fourths, before painfully struggling upwards by way of semitone steps, in search of an unfulfilled resolution. In just three bars of music, Wagner had encapsulated the gnawing ache of longing. The 'Tristan chord', as it came to be known, and its associated chromatic phrase, continues to underpin the harmonic structure of the entire score, returning with renewed urgency towards the end of Act One, when Tristan and Isolde drink the fatal love potion, and emphasizing the desolation of the opening of Act Three, as the mortally wounded Tristan awaits Isolde's arrival. Not until the very end of the opera does the Prelude's yearning phrase find resolution on the radiant chord of B major, after the transfigured Isolde - like Brunnhilde at the end of Götterdämmerung - has taken centre stage for her ecstatic Liebestod, in which she seeks mystical union with her lover in death.