Im playing a good deal of baroque flute—I have one tuned to 440. I have heard that 415 is more in use by baroque flute players and baroque ensembles. Anyone who knows about such things, is this accurate?
I’d say that is true, although not universally so. There are a few different pitch standards used, including one as low as A=392. I think A=415 is the most widely used though, but I’m no expert on this.
Interesting, a D flute at A=392 hz is basically the same thing as a C flute at A=440 hz. And a D flute at A=415 is basically the same thing as a C# flute at A=440 hz.
A415 is probably most popular for period-instrument ensembles. Similar to Paddler’s comment about 392 being basically playing in C, 415 is playing in C#. It’s popular because it’s convenient for harpsichords – those designed to play in different pitches have a mechanism that effectively allows the keyboard to shift over by one note at a time. Personally, I love A415 and never use the A440 midsection in my traversi.
Quantz pointed out that there were flutes in his time that had as many as seven different corps des rechanges that allowed players to move around to different areas to account for differences in local pitch (pretty much the organs).
Here is an example of a Quantz flute with 6 different upper sections to select from when playing at different pitches. It is interesting that they use just a single right hand section to work with all six of them! 
https://www.loc.gov/resource/music.musdcmflute-0916-001/?sp=2
The six corps de rechange on this original Quantz flute play at A = 387, 392, 397, 402, 407, and 412, so nowhere near A = 440 hz. Supposedly, the flute was intended to be played primarily using the lower pitched bodies.
You can read more about Quantz flutes here:
And all of them playing south of 415. So where did that fit in?
Best wishes.
Steve
The pitches were regional. Each city had an organ that played at a whatever pitch the pipe maker designed it for. I don’t know whether each maker had a particular pitch he designed for, or whether he did it by ear and so all of his organs were around the same pitch but not exactly. This flute may have been made for an itinerant fluter who traveled around a region with an organ maker whose ear liked A around 400 hz, but not each instrument turned out the same.
Patrick Olwell talks about this in an interview from many years ago.
What he was discussing was how so many of the surviving old English old-system 8 key flutes were built to A-452, which Irish fluters played at modern pitch (A=440) by simply drawing out the tuning slide.
Then when the boom in Irish fluteplaying led to a boom in Irish flutemaking some of the new makers copied those trusty old 452 flutes, just making them with a longer headjoint.
This of course resulted in flutes with faulty scales which the makers then had to goat-rope into tune.
Patrick noted that in the Baroque period, when flutists had to deal with playing every gig at a different pitch, they didn’t carry around a bunch of different headjoints! Instead they carried around a bunch of different middle-joints (upper-hand joints). This is because the upper-hand tuning is far more impacted than the lower-hand tuning when you change overall length.
So that’s what he did when copying a great-playing vintage flute built to 452, he would lengthen the middle joint rather than the head.
That particular flute was made for the King of Prussia, Frederick II. Careful, he might not appreciate being referred to as an itinerant fluter. 
When you study a lot of those older English 8-key flutes, what you find is that many, if not most, of them were originally made for a tuning standard that was not too far off A=440, but that as pitch standards rose in the UK, eventually peaking at A=456 hz, makers of the time cut down the head joint and used long tuning slides in order to allow the flutes to play at much higher pitches than their original design intended. There certainly were some flutes designed specifically for high pitch, but they are actually in the minority, even among English makers.
One way you can know this is by examining the tuning of these flutes at various different tuning slide extensions and looking to see where the flute is best in tune with itself. Many old English flutes are clearly best in tune with themselves at the lower pitches of their intended range, often requiring well over 20 mm of tuning slide extension to have internally balanced tuning. With the tuning slide closed their internal tuning exhibits some extreme flattening trends as you get closer to the foot notes. Had they been tuned for higher pitches and you then lengthened the slide to enable them to play at A=440 hz, what you would observe is that the right hand and foot notes would be way too high. It is quite rare to find an antique flute with those tuning characteristics.
But the problem with playing them with the slide extended this is that the huge cavity introduced into the bore by the slide being open this far interferes with the tuning of some notes and reduces the strength of tone of many others. Making a longer head so that the tuning slide can be closed while preserving the same overall distance between embouchure and tone holes improves matters considerably. Basically, what you see is that the bore profile, reamers, tone hole layout, etc were all designed for lower pitches originally, but then as the pitch standards rose in the UK, many manufacturers simply shortened the head and lengthened the tuning slide to try to produce a flute that could work across a wide range of pitches. This approach is much easier than making new reamers and redesigning the whole flute. But this approach simply doesn’t work well, for the reasons outlined, and because the internal tuning is actually even worse at the highest pitches. The tuning slide approach is certainly less effective than corps de rechange, but it is cheaper for the maker, and allows the marketing department to advertise a new technological feature (the tuning slide) to drive more sales. All in all, it is just a step back in flute technology.
The history of changes in pitch standards is quite interesting to study. Eventually, the drive to higher and higher pitch standards in Britain caused problems, especially for singers, and they decided to try to standardize the pitch to something sane, and lower. The French had already gone through that process and had settled on diapason norma pitch, which is A=435. But they defined that standard at a temperature of 15 degrees C, which was much colder than typical concert halls in Britain. The British eventually defined their Philharmonic Pitch standard to be A=439 hz, which is precisely what A=435 hz works out to be a typical concert hall temperatures of around 20 degrees C. In other words, A=435 at 15C is the same standard as A=439 at 20 C.
The problem with A=439 hz is that 439 is a prime number and that made it difficult to deal with using the technology of the time. So they rounded it up to A=440 hz as 440 was a much easier number to deal with.
A=415 hz comes from A=440 simply by being a semitone lower. I.e., 415 hz is the frequency of the note G# under the A=440 hz pitch standard.
A=392 hz comes from A=440 simply by being a whole tone lower. I.e., 392 hz is the frequency of the notes G under the A=440 hz pitch standard.
Selecting these particular two lower pitch standards to approximate the kinds of pitches used in the baroque period simply allowed harpsichords that were tuned to A=440 hz to be retuned quickly either a semitone or a tone lower, and then used for baroque performances, as Chas mentioned earlier.
Heh heh, whenever the matter of pitch versus location comes up, I’m reminded of Robert Sidney Pratten. He wasn’t just Professor of Flute, he was also a virtuoso on the newly fangled concertina. When he embarked on a grand tour of concert venues in the UK and Europe, the story goes that he had to bring 7 concertinas to cover the range of pitches in use at the time.
Unfortunately I don’t believe the story tells us what he packed in the way of a flute.