On the day we started tuning the Sakuras’ piano again, the twins’ mother told me Kazune never found practising the piano a strain, however long she sat at the keyboard.

‘She says she never gets tired, no matter how long she plays,’ Mrs Sakura said, smiling sweetly.

‘Being able to play that long is a talent in itself,’ Mr Yanagi put in.

And I totally agreed. It wasn’t as if Kazune was forcing herself to play. Someone who can put in hard graft without resentment is at a distinct advantage. If you find something difficult or boring, you’re more likely to want a major return on your investment of time and effort and then be dissatisfied with the outcome. But because Kazune was able to slog away at her practice and enjoy it, the possibilities before her were beyond imagining.

There was a decisiveness to how she approached the piano that I envied. Seated at the keyboard, it seemed she was capable of almost anything.

Personally, I didn’t know where to focus my efforts, which is why I did things in such a haphazard manner. But there in the office at that early hour, I opened up the lid of a grand piano (the same model as the one in Kazune’s home) and set myself a clear goal: to retune this one instrument using the well-tempered system.

When tuning a piano there are several schemes available for dealing with the twelve musical intervals of an octave – the do, re, mi, fa, so, la, ti, do, plus the semitones. The two main methods are well-tempered and equal-tempered tuning.

Equal-tempered is a system based on logic, dividing an octave into twelve equal parts, and is the tuning used for nearly all pianos. There are a few issues with it, since adjacent notes are tuned to have an equal interval between them even though, strictly speaking, they are not a regular distance apart in terms of resonance, so when you combine sounds they result in a certain muddiness. For instance, with chords, the mi notes in do-mi-so and in la-do-mi are intrinsically different pitches.

By contrast, well-tempered tuning prioritizes this resonance of sounds. It regulates things so that the frequency of each note is a ratio of whole numbers in mathematical terms. When you put several sounds together, the simpler the frequency ratio, the more the resonance is pleasing to the ear – which is why chords sound so beautiful on a piano tuned to the well-tempered system. There is one significant weakness to this method, however, namely that since the interval between each note is different, it can’t be used when you change key.