A being 440 is irrelevant. A could be anything.
Modes were used as major and minor are used today at some point in history.
I am not a music historian. But its part physics. We are talking about sound waves. And there are always overtones in a sound. These overtones tell us what is constant and what is dissonant. The first overtone is the octave. So naturally an octave is the most constant interval, ignoring an unison. This makes alot of sense. The air waves are in sync with each other.
Then we have the tast of dividing the octave, how do we do that? It is interesting that several musical cultures descided to do it the same way independently.
The second overtone is the fifth, if you take the 440 note, which you call A, then the first overtone is the 880 freqeuncy. The second one is a freqeuncy 3/2 times higher. This is the fifth. This is 1320 Hz, which is an overtone of 660. So it makes sense that those notes are constant.
You can continue this, you get a major third, then an octave and a fifth again, I think, and then it gets a bit messy. You get something that approaches the Major 7th. Of course a major seventh isn't a nice constant.
You can also look down. 440 is the fifth of 293 Hz (D), or its octave 586,67
So we have the main note A(440) we have the octave A2(880), we have a fifth E (660), we have a major third(the actual overtone is closer to a major third than a minor third), 495(C#) we have a major seventh, 825(G#) and the fourth, because the A is an overtone of the fourth, 586,67(D).
So
A C# D E G# A
This is the pentatonic major scale. Invented by Pythagoras(or at least, he is credited for it, maybe one of his students). Also note that this scale is created by stacked fifths, the 3/2 interval, the second overtone. Its either done that way, or the way I did it, by measuring the overtones.
This scale was not only invented by Pythagoras, it is also used in Chinese music, in African music, in Indonesia, in Scottish and English folk music, all over the place.
The pentatonic scale is an universal truth, just like the Pythagorean triangle theorem. It was already there, a byproduct of the laws of the universe.
If you fiddle around with the pentatonic scale you will notice it is a very simple and melodic scale. You can end on any one and make it sound ok.
Now, for some reason the number 7 is a 'magical' number. I am not sure why, but for some reason it is best to divide the octave into 7 parts(so eight notes if you count the octave itself too, hence its called 'octave'). We added two notes, a major second inbetween the unison and the major third, where there was a gap, and in the other gap, inbetween the fifth and the major seventh, we added a major sixth.
I am not sure how they actually got those notes. But now we have the major scale. And to any human, using 7 notes plus the octave just sounds the most perfect and most natural.
But western music has a problem. Lets take the first note, 100 Hz.
So then the natural overtones are 200, 300, 400, 500. But if we are going to stack fifths then we get 100, 150, 225, 337,5 and then 506,25. Thats 6,25 Hz too much. Its 81/80 too much. This means you can only tune an instrument and get the perfect overtones build on only one frequency.
To tune a piano or any kind klavier/keyboard in a way all keys sound equal in tune they tuned the fifths slightly lower than dictated by the universe, and the thirds slightly bigger. This way you can make the piano sound in tune for any key. Otherwise you have to tune it to one key, lets say A major. This is where things like Well Tempered and Equal Tempered come from. This is why Bach composed a prelude and a fugue in every key, because which his new keyboard it was possible to sound in tune in every key.
There is another problem.
If you start stacking fifths on Gb then after 12 fifths you get a F#. That note is (3/2)^12 times higher. But if you start stacking octaves on Gb then after 7 of them you will get a Gb that is 2^7 times higher. Now (3/2)^12 is 129.75 and 7^2 is 128
This problem is alot more subtle and was solved in the same way.
The system we use in the west today does not use the 3/2 fractions we got from the overtones. No, we now divide the octave into 12 equal parts. So the twelfth root of two, which is 1,05946 and alot more numbers. Its kind of like a musica Pi.
With this number, we can make every key sound kind of in tune with the natural overtones. This number created the chromatic scale. Now, then you can pick any combinations of intervals and create new scales.
Look at this site for more info:
https://www.phy.mtu.edu/~suits/scales.htmlWhat did they do in other cultures?
Not every musical cultures uses chords and harmony like we do. Most of them don't have chords or keys.
Lets take south indian carnatic music, which I know some stuff about. They still use what is called the just scale. Their fifth is 3/2 and not 7 times the twelfth root of 2. The difference is 3/2 = 1.5000 and 1.49831.
Carnatic music is based on singing. There is no standard for notes like A = 440 and concert C is 273. A singer has a range and he needs to pick a base pitch which he can comfortably sing an octave and a half above and half an octave lower.
They don't have keys, there is only one key, and what the actual pitch is is different for every person. The instruments tune up with the singer, which is usually the solist.
So they have no use for a tempered scale. All their scales are just scales. Also, all their scales have a root and a perfect fifth. Diminished fifths and augmented fifths aren't used. Now they have a mathematical structure to fill in all the other notes. Like I said before, they also prefer 7 note scales, probably because of the way our brains work.
They have 72 seven note scales, they call them melakarta ragams. All have a perfect fifth on the base note. Thirthy-six of them have a perfect fourth, and the other 36 have augmented fourts.
So they have scales that mach ours. What they call Sankarabharanam( or Dheera-Sankarabharanam) is the major scale. As you see, our name is a bit simpler. The Sankarabharanam ragam is a popular one.
Another aspect to their music style is the use of microtonal ornamentations. Because their music is more melodic and doesn't have a strict system of the notes to use they use alot of sounds inbetween their notes. This is very normal and very awkward to the average western ear. It is actually very colourful and melodic.
The same is done in the middle east and in a whole lot of other musical cultures. Even the blue note in the blues has this. This is why blues music is better played on guitar or sung instead of played on piano. On a guitar you can bend notes 'out of tune' to hit the awkward blue notes.
As for modes and major and minor. People started to change the modes at places so they could get better and more perfect cadences. So the modes kind of evolved into two subforms. Major and minor.
The reason that A is called 440 is because A had to be some note. It would have made no difference to have called it 441 or 430 or 457.
Well, this is alot of info. If I was incorrect or incomplete, please correct me.