“THE most erotic music ever composed.” This was how one critic described Richard Wagner’s “Tristan and Isolde”, which in 2015 marks the 150th anniversary of its first performance. The work is divisive. Many 19th-century critics thought little of it; some even mocked it. But it had an enormous impact, ushering in the move towards atonality that found its most extreme expression in the work of composers like Arnold Schoenberg. Today it is the most famous opera by Germany’s most famous opera composer. But why all the fuss?

The opera traces the story of two lovers, Tristan and Isolde (painted above by August Spiess in 1881), who fall madly in love after drinking a potion. The plot is forgettable; the music is not. The magic starts with the prelude, a ten-minute musical introduction that structures the rest of the piece. The opening chord, known to music buffs as the “Tristan chord”, shocked the 19th-century listeners who heard it first.

Normally, in classical music, dissonance resolves into consonance; tension melts into resolution. But the Tristan chord, and the music that follows it, dispenses with such convention. It tempts you to listen out for a particular chord (A minor). But it never comes. Instead other dissonant chords emerge, which while not unpleasant, do not provide any relief. By the end of the prelude, the listener is left unsatisfied and waiting for more.

The prelude sets the tone for the rest of the opera. Western music composed before “Tristan” tends to be tightly structured. Composers of the classical period of the late 18th century, such as Haydn and Mozart, often opt for four- or eight-bar phrases with a clear beginning and end (the opening of Beethoven’s first piano sonata is a prime example). There are often clear, easy-to-follow tunes, so the listener knows where the music is going. Not so in “Tristan”. Overlapping melodies flow for dozens of bars; humming the opera is tough.

In fact, the music was so unusual that, at first, musicians struggled to play it, says Laurence Dreyfus of Oxford University. The first performance was supposed to be in Vienna in 1862. But after more than 70 rehearsals, the production was cancelled. The singers could not find their notes and the orchestra struggled to keep time. Only by 1865, with its premiere in Munich, had the musicians caught up with Wagner and his “Tristan”. The critics were less convinced: the singing was “nothing but screaming and shrieking”, fumed one, while the orchestra indulged in the “most outrageous discords”.

Why did Wagner choose such an unusual style? Partly it was the result of a new outlook on life. He discovered the work of Arthur Schopenhauer, a German philosopher, in 1854. Schopenhauer believed that people were driven by unachievable desires; the chasm between what they want and what they can get is immiserating. Wagner was inspired by such pessimism. For a man prone to fleeting romantic liaisons, love was always unsatisfying. And he came to believe, again thanks to Schopenhauer, that music was the best way to express such complex emotions. That forced him to abandon temporarily his work on the “Ring” cycle, where the music is really a complement to the drama, and instead knuckle down to “Tristan”, where the music is clearly the main event.

“Tristan” is all about dissatisfaction. The love between Tristan and Isolde is so intense it cannot possibly be realised in the real world; their only option is to die together. To represent the pain of their unachievable desire, Wagner had to dispense with conventional harmony. He also could not offer the audience neat chunks of music that were easily digested. He wanted them to feel as anxious and confused as the protagonists.

Wagner uses musical tricks to toy with his audience, says Mr Dreyfus. The opening phrase of the final act turns out to be another form of the Tristan chord, only darker and even more melancholy, which emphasises the tension. One of the most famous scenes, in the second act, is an amorous encounter between the lovers that is brutally spoiled right before the climax. More than two hours into the opera, this violent coitus interruptus is hard to bear. Two conductors have died leading the second act.

Some productions add to the pain. Traditional Wagnerian performances tend to be rather silly spectacles, with lots of swords, cloaks and horned helmets. One of the most recent productions of “Tristan”, which was directed by Christof Loy at the Royal Opera House in London and which closed shortly before Christmas, was less bombastic. Much of the action took place in just one corner of the stage, a chair at times being the only prop. The spartan set removed even the lightest of relief from the listener’s experience.

Mercifully, Wagner rewards his audience at the end of the third act. After Tristan dies, Isolde begins a long solo that leads to the opera’s conclusion. Five hours after the first Tristan chord demanded it, a quiet, extended and wholly unexpected major chord provides a glorious resolution. It was, said Friedrich Nietzsche, “a spine-tingling and blissful infinity”. The orchestra fades out, and Isolde, too, is dead.