On Christmas Day, 1870, the sounds of Wagner’s Siegfried Idyll wafted up the staircase of a villa in Switzerland. This beautiful piece – which has become arguably the composer’s best-loved orchestral work – was a gift from Wagner, written and rehearsed in secret, for Cosima, his second wife.

The premiere might have been a private and personal event, but Wagner had ensured it would be memorable, enlisting 15 (it’s thought most likely) instrumentalists from the prestigious Tonhalle Orchester Zürich, including the famous conductor Hans Richter on trumpet. ‘Music was sounding, and what music!’ recalled Cosima of the day. ‘After it had died away, R… put into my hands the score of his “Symphonic Birthday Greeting”.’

Yes, birthday greeting. For although this was a festive occasion, the music in fact marked Cosima’s birthday, which fell on 24 December but which she always celebrated on Christmas Day. The pair had special reason to mark 1870, too, as after a complicated six-year relationship and three children together, they had finally married on 25 August at the protestant church in Lucerne.

For them, it was a respectable seal of approval on a scandalous relationship. Cosima and Richard had first encountered each other in 1853 when she was a teenager, and he came to visit her father, the composer Franz Liszt. In the 1860s they fell in love. Wagner was still married to Minna Planer – although his extra-marital dalliances had effectively ended this relationship – while Cosima had married the conductor Hans von Bülow. A great champion of Wagner’s music, it was, ironically, thanks to Von Bülow that Wagner and Cosima started to spend time together. In November 1863, ‘with tears and sobs,’ wrote Wagner, ‘we sealed our confession to belong to each other alone’. They embarked on an affair.

In 1865, King Ludwig II of Bavaria became Wagner’s patron. At the composer’s suggestion, Von Bülow was taken on as ‘royal pianist’ while Cosima ostensibly became Wagner’s secretary. They all moved to Munich. By then, the penny had dropped for Von Bülow, his suspicions cemented rather belatedly by the birth of Isolde, Cosima’s first child with Wagner, in April 1865.

Yet Von Bülow conducted the premiere of Tristan und Isolde that June, and legally accepted Isolde as his own. Public denials – including a royal decree – of Wagner and Cosima’s relationship followed, but privately it continued. Eva was born in 1867, Siegfried in 1868. By then Wagner had been forced into exile in Switzerland and moved into Tribschen, the villa by Lake Lucerne. Cosima joined him for good in 1868, applying for a divorce which came through on 18 July 1870.

Which brings us back to the Siegfried Idyll. In its original form, this single-movement piece of around 20 minutes was for five woodwind, three brass instruments and a string quintet. Wagner drew on the final act of his opera Siegfried for some of the musical material, and also worked in personal references, including the German lullaby ‘Sleep, baby, sleep’. And while Wagner had intended his tender, warm Idyll for Cosima’s ears only, in 1878 he found himself short of money. He expanded the orchestration to 35 parts, and sold it to the publisher B Schott.

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