Congratulations, you have stumbled into the exciting world of historically-informed performance! As you might already guess, this can be a somewhat fraught concept. There are fiery debates over simply matter of articulation or tempo in a single movement of a single piece, never mind the entire breadth of what we call "classical music." But it's worth diving into what exactly we mean when we talk about these things.
First of all, the idea that we actually want the music to sound as "historically accurate" as possible is not necessarily a universal one. Classical musicians play music in a way that isn't quite "accurate" all the time; in some instances there's even significant accepted tradition to do things a certain way. This was especially true before the significant rise of the "Historically-Informed Performance" (HIP) movement in the mid-20th century. It was de rigeur, for example, for big orchestras built for Romantic and Modern-era music to play Bach in much the same way, as recordings by the likes of Otto Klemperer and Leopold Stokowski can attest. Stokowski in particular is renowned for his orchestral arrangements of Bach that don't really pretend to sound like what Bach "would have heard."
There are plenty of historical debates about music history and performance practice, but as I mentioned above, the idea of HIP really starts gaining steam after World War II. Concentus Musicus Wien was founded in 1953 by Nikolas and Alice Harnoncourt, and is often held up as the pioneering group for playing music on period instruments and in period styles. The 70s saw a massive explosion in these kinds of groups, with the likes of the English Concert, the Academy of Ancient Music, Musica Antique Koln, and tons of others coming onto the scene. All of them purported to play the music of the 17th and 18th centuries as you would have heard in that time, through significant research and playing on either actual period instruments or modern-built replicas.
I'll give a bit of a rundown as to how we get there, before ultimately questioning why it matters. I'll start with something I just touched on...
Instruments
If you go to see a symphony orchestra or chamber music concert today, there's a good likelihood that they'll be playing Mozart/Bach/Beethoven/etc. on instruments that did not exist in their modern form when those composers wrote whatever piece you're listening to. This is particularly notable in wind and brass instruments, which went through considerable changes in the late 18th and 19th centuries. The flute, for example, went through a revolution in the mid-19th century with Theobald Boehm's system becoming the standard by the end of the century. If you hear a Mozart flute quartet or Quantz flute concerto on a Boehm system flute (the shiny silver ones that every band kid and orchestra player has), you are not hearing the music "as it was originally performed." The instrument just sounds different, as you can hear in these two performances of the same Bach flute partita, one on a modern Boehm flute and the other on a Baroque-style flute akin to what Bach would have had to work with.
So, in order to sound "historically accurate" you have to use the right instruments. Luckily, we have a lot of old instruments available to us. Some are in playable condition, some aren't, but there are enough around for modern makers to make fairly good and "accurate" copies of the instruments. Now, why did I put "accurate" in quotation marks? Well, a modern replica will almost never be exactly the same as the instrument it is copying. Different pitch and tuning standards (more on that later) in the olden days mean that an exact copy would not fit in with a lot of ensembles. Sometimes, makers will "fix" flaws in the old design for a more desirable sound, without going as far as Mr. Boehm and others did. And while older instruments were made out of things like wood and ivory, today we have a lot of synthetic materials that are often used in instruments (especially faux ivory, due to legal issues in transporting the real thing).
Von Heune is an excellent example of current makers of older designs. Note that their flutes are labelled with the name of the maker of the original flute they copied. For example, "Grenser" flutes are popular 18th century-styled instruments patterned after those made by August Grenser from Dresden.
Pitch and Tuning
Today, most people play in what we call A440, meaning that the A above middle C is a note that we define by the frequency of 440Hz. This is what symphony orchestras and other classical ensembles will usually use as a baseline. It's obviously quite important to have everyone in tune, so picking a standard pitch is something that everyone agrees on. This has not always been the case, however. There are a number of pitch systems that have been used throughout the years, meaning that for our music to be "historically accurate," we've got to figure out how to make it sound in the right pitch.
Not only that, but the idea of "in tune" is a somewhat more complicated one than you might think. I won't get too far into the weeds of music theory, but basically the system most people use now is a compromise. It's called "equal temperament," which means that essentially we fudge each note a little bit so that we're pretty much in tune for every single key, rather than very nicely in tune for one key and very poorly out of tune for everything else. Again, this gets complicated quickly so I'll just let you listen to a demonstration of "just intonation" (the one that is perfect for a single key) and "equal temperament" in a famous Bach tune. See if you can catch where things start sounding a little odd once we get out of the home key, and how equal temperament tries to change that.
Now, luckily for us the ideas of has been a vexing problem for a long, long time, and so we have all kinds of writing on it. We have acoustic scientists like Vincenzo Galilei writing up their experiments and research on tunings (he had his son Galileo help him out, teaching the kid a thing or two about the scientific method). We people like Michael Praetorius commenting on the fact that the "standard" pitch that everyone played at around him in the 16th and 17th centuries seemed to be rising, to the chagrin of singers forced to hit higher and higher notes. We have old instruments whose characteristics can tell us what pitches and tuning systems were being used when they were built. And we have clues the composers themselves left, like Bach's famous work The Well-Tempered Clavier, which Bach meant to be played in a sort of cousin of equal temperament.
Now, we still ultimately have to agree if we're all going to play together, so players of older music often fudge a bit. Baroque music is often played at A415, one half step down from A440, although you'll also see staunch advocates for A392 (another half step down) or other pitch standards. Is this "historically accurate?" Well, maybe, but we definitely don't want to be out of tune!
Technique
So, we've got our instrument, we've figured out how we like it pitched and tuned, now how do we play it? Well, luckily, we've got a lot of material to work with! Go to a music store today and you'll be able to pick up all kinds of books on how to play guitar, how to play Taylor Swift on ukulele, how to play the blues on the harmonica, etc. There's nothing new under the sun, and you'll find that exact same genre of book for many instruments throughout history. Carl Philip Emmanuel Bach wrote an "Essay on the True Art of Playing Keyboard Instruments" that proved immensely popular, for example, and his colleague Johann Joachim Quantz wrote a similar treatise called "On Playing the Flute." Today, we can read through these and learn a lot about how musicians played those instruments in the 18th century.
When performing music written before recordings were invented, classical musicians can never be 100% sure that they are performing it the way it was originally performed. In fact, in the majority of modern performances these pieces of music are not performed precisely as they were when they were written. Classical music has a constantly morphing tradition of performance practice where aspects of performance style can change even in the span of a few decades. If we compare a recording of a Gustav Mahler symphony from the 1930s to a contemporary one, there are subtle but crucial differences in the way that the orchestras and the conductors approach the piece.
This is a recording of the conductor Bruno Walter leading the Vienna Philharmonic in Mahler’s 9th Symphony made in 1938:
(Bruno Walter was a conducting student of Gustav Mahler and learned the conducting style that was intended for his music from the source. While Walter was an artist in his own right with his own interpretational ideas, he has a special authority in the interpretation of Mahler’s music.)
For a comparison, here is a performance from 2020 of Myung-Whun Chung leading the Concertgebouw Orchestra:
https://youtu.be/7NKvBNliyN8?t=110
Even in just the opening minute of music there are already several differences: The strings have a different, more resonant sound. The sounds of the violins and horn are combined by the players to create a different color than in the 1938 recording. Most notably, the modern orchestra sounds significantly louder.
Mahler is a comparatively recent composer to find these differences in performance practice. If we were to compare performances of a piece by Wolfgang Amadeus Mozart, one from 1938 and the other from 2020, the differences would be even starker.
Tastes in performance style change over time and for most of classical music’s history, performers have aimed to please contemporary preferences rather than fulfill an abstract idea of “faithfulness” to the first performances after they were written.
From the 1940s into the 1970s, a new approach to performing classical music arose called the Historically Informed Performance movement. Rather than updating the performance style of older pieces of music to fit modern tastes—as most did/do—several prominent musicians, including Nikolaus Harnoncourt, Gustav Leonhardt and John Elliot Gardiner among others, began to perform music based on extensive historical musicological research. They believed that music written before 1800 had become distorted in modern performing styles and that the music’s original impact and intention was being lost. They strove to recreate the original performing styles of works by many prominent composers, ranging from Josquin de Prez, to J. S. Bach, to Ludwig van Beethoven.
They recreated these historical performance styles by consulting writings about music that were contemporaneous to the pieces that they were playing. Throughout history, musicians and authors have written extensively about music. Some of these writings are general descriptions of instruments and ensembles. Some are literary descriptions from writers like Boccaccio and Shakespeare. Others are personal recollections from musicians and audiences found in memoirs, diaries and personal correspondence.
Most informative among these sources are the multitude of musical treatises that survive from past centuries. Many of these are performing manuals that were meant to teach students how to play a particular instrument or a particular genre of music. Others are theoretical texts that mention performance practices in passing.
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