When orchestra members tune their instruments before a performance, it almost always sounds the same. That’s because across the world, most orchestras tune to the same A note, using a standard pitch of 440 hertz.
This is the result of international standards that have been in place since the 19th century, according to WQXR, a classical music radio station in New York City. Currently, standard tuning frequency is set by the International Organization for Standardization (ISO), an international group that makes recommendations on everything from what safety labels should look like to how big the hole in a pen cap should be. A standard called ISO 16, first recommended in 1955 and confirmed in 1975, “specifies the frequency for the note A in the treble stave and shall be 440 hertz.”
The ISO didn’t pull that frequency out of thin air. During the Industrial Revolution, a rush toward standardization and universality led to multiple international meetings that aimed to bring orchestras all over the world to the same pitch. Standardizing pitch had important ramifications for the international music scene.
Historically, the pitch that orchestras tuned to could differ wildly depending on where the musicians were playing. “In the course of the last 400 years in Europe, the point that has been considered ideal for a reference pitch has fluctuated by some 5 or 6 semitones,” musicologist Bruce Haynes explained in his book, A History of Performing Pitch: The Story of ‘A.’ In the 17th century, a French performer might tune his or her instrument a whole tone lower than their German colleagues. The standards could even change from one town to the next, affecting how music written in one location might sound when played in another.
As a writer for London's The Spectator observed in 1859, “It is well known that when we are performing Handel's music (for example) from the very notes in which he wrote it, we are really performing it nearly a whole tone higher than he intended;—the sound associated in his ear with the note A, being nearly the same sound which, in our ear, is associated with the note G.”
In the 19th century, a commission established by the French government tried to analyze pitch across Europe by looking at the frequencies of the tuning forks musicians used as their reference while tuning their instruments. The commission gathered tuning forks from different cities, finding that most were pitched somewhere around 445 hertz. Over the years, due to bigger concert halls and more advanced instruments, pitch was rising across most orchestras, and instruments and voices were being strained as a result. So the commission recommended lowering the standard to what was known as “the compromise pitch.”
In 1859, the French commission legally established diapason normal, the standard pitch for the A above middle C, at 435 hertz. (The music world would still be debating whether or not pitch had risen too much more than a century later.) Later, 435 hertz became enshrined as a standard elsewhere, too. In 1885, government representatives from Italy, Austria, Hungary, Prussia, Russia, Saxony, Sweden, and Württemberg met to establish their own international standard, agreeing on 435 hertz. The agreement was eventually written into the Treaty of Versailles in 1919.
But not everyone was on board with 435 hertz. The Royal Philharmonic Society in London believed the French pitch standard was pegged to a specific temperature—59°F—and decided to adjust their pitch upward to compensate for their concert halls being warmer than that, settling on 439 hertz. Meanwhile, in 1917, the American Federation of Musicians declared 440 hertz to be the standard pitch in the U.S.
In 1939, the International Standardizing Organization met in London to agree on a standard for concert pitch to be used across the world. A Dutch study of European pitch that year had found that while pitch varied across orchestras and countries, the average of those varied pitches was around 440 hertz. So it made sense for the ISO to choose A 440. Furthermore, radio broadcasters and technicians like the BBC preferred A 440 to the English A 439 because 439 was a prime number and thus harder to reproduce in a laboratory.
World War II delayed the official launch of the 1939 ISO agreement, but the organization issued its A 440 decision in 1955, then again two decades later. A 440 was here to stay. That said, even now, pitch does vary a little depending on the musicians in question. The Vienna Philharmonic Orchestra notably tunes to 443 hertz rather than the standard 440 hertz, for instance. While A 440 may be the official “concert pitch” across the world, in practice, there is still a little wiggle room.
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