Hot on the heels of their world-changing album Sgt Pepper’s Lonely Hearts Club Band, The Beatles decided to make their own movie. While the result was a catastrophic mess that only the band themselves failed to see, which was an appalling disservice to the art of filmmaking, it produced some brilliant songs that are often overlooked in the Beatles’ discography.

One of the best tracks to come out of the Magical Mystery Tour project was Paul McCartney’s ‘The Fool on the Hill’, which sees its composer come as close to John Lennon’s philosophical approach to songwriting as he ever did on a Beatles record. The prominent use of flute instruments is also a unique feature of the song among the group’s back catalogue.

Orchestral flutes are the main backing instrument throughout the track, alongside McCartney’s piano part. But alongside the three professional flautists, McCartney himself steals the show soloing on another woodwind instrument at three different points in the song. These solos were actually recorded prior to the orchestral overdubs, which producer George Martin says were added later to make the record “sound a little more professional”.

Meanwhile, McCartney’s woodwind part was entirely his own initiative, recorded at his insistence. Listening to the song today, it feels like an inspired decision. Yet it was one that Martin took a while to get on board with.

So what was Paul playing?

McCartney is generally credited on ‘The Fool on the Hill’ as playing a wooden recorder, a type of flute with a whistle mouthpiece also referred to by the technical term “fipple flute”. However, during a 1969 episode of John Gilliand’s radio documentary Pop Chronicles, Martin specified that it wasn’t actually a recorder being played on the song.

Early in the recording process, according to Martin, “Paul brought along his little toy whistle, his little penny whistle, that he was experimenting with.” As opposed to a wooden recorder, a penny whistle, otherwise known as a tin whistle, is a slight metal instrument that typically has exactly five finger holes. The modern version of the instrument was invented by Robert Clarke in the northern English city of Manchester mid-way through the 19th century and was most commonly seen as a children’s toy during the 20th century.

To Martin’s displeasure, McCartney said he “wanted to try and use it” on the latest Beatles recording. In the end, the producer relented, though. “He’s on the record, that’s his penny whistle,” Martin confirmed. “And we double-tracked it to make him sound a bit better.”

In fact, the childlike innocence invoked by the penny whistle’s sound perfectly mimics the apparent naivety of the “fool” described in the song. As usual, McCartney’s musical instincts were on point, and his willingness to experiment in the studio led to an inspired piece of instrumentation.

His penny whistle belongs in the pantheon of unexpected sounds on Beatles song, without which their recorded output just wouldn’t feel quite as Beatlesy. Just like the fool on the hill himself, four lads from Liverpool without any formal musical education often saw things classically trained musicians just couldn’t see.