That actually made sense to me @joergkutter, thanks for taking the time to write this. Let’s see if someone has something to add
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A piece could absolutely be written in B dorian, but the octave is irrelevant. Octave will never be indicated in the key of a song. If pitch level is important to the song, it has to be indicated in the notation of the music and the instrumentation.
Any triad with the intervals Root, flat third, perfect 5th is a minor triad, and will allow for a consonant resolution. Thus, B dorian is a minor key because the triad built (using the root third and fifth) in that mode gives you a minor triad. Because B dorian shares a key signature with A major (A ionian) you would write the key signature for A major in the piece. You would communicate to the musicians that the piece is in B dorian through the music - the fact that the music resolves to B dorian. Context is the only way that you know what musical home the piece is in. This is the case for any piece of music, and any of the 7 modes.
The key signatures are named after their most common usage which is still the major scale/ionian mode.
For example, the famous jazz song “So What” from the Miles Davis album “Kind of Blue” is in D dorian. The songs musical focus is D dorian, the resolutions are all to D dorian. It begins and ends on a D minor chord. The song (on paper) is written with the key signature of C major - no sharps, no flats, because those are the notes in D dorian.
I think the problem may be one of terminology.
Key signatures could (and probably should) be named for the notes that make them up, not for a single note and system that the key signature creates.
Meaning - The key of A major has 3 sharps, creating a 7 note pattern that repeats through all octaves infinitely. It could be called The Key of 3 Sharps. Or the key of ABC#DEF#G#, or it could be called the 3-o-clock Key (because of where it sits on the circle of fifths). A less harmonic-centric name for the Key Signature might help in understanding the key of 3 sharps to be simultaneusly the key of:
A major (ionian)
B dorian
C# Phrygian
D Lydian
E Mixolydian
F# minor (aeolian)
G# locrian
Because the most referenced, popular and historically significant pieces written with this system favor the major or minor scale (ionian or aeolean) the nomenclature for key signatures remains simplified as: the key of 3 sharps is the key of A major.
You’re absolutely correct that music theory is a mess. It is a system created for the aid in studying great compositions of the past. It comes up with terminology and rules and descriptions to describe things that already happened, and in the meantime musicians are bending and breaking all the rules and pushing music into new sonic places.
Learning music theory is frustrating in this way.
It starts out with - here are the rules and names for things, and you can’t break them or its wrong.
Then you go to your next year of theory and the rules change: OK, now you can break all those first rules, but here are the new rules and names for things, and these are the ones you can’t break.
Then you get to 20th century music and the rules get ridiculous. OK, so this composer made up his own rules, and to compose like this, here are the rules to follow. Also, all the rules can be broken now, so long as you know they’re there.
And then, finally… you end up in theory Nirvana:
If it sounds good, it’s right.
Not sure how this lands or if it helps.
I sure do love talking about this stuff though.
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@Gio maybe you can split these posts into a topic of their own? That would make them easier to find again.
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Oh that’s my fault. I meant to write Dorian b2 (flat 2nd) and not a Dorian with a B2 tonic 
(I still have to read your post with full attention, but didn’t want this detail to linger)
I agree. There’s gold in some posts. That maybe don’t fit the “ah hah!” thread but are very valuable theory wise.
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Ha! Maybe? Maybe not. Check out some of the music by Anton Webern, Charles Ives, Arnold Schoenberg, Elliot Carter (in the classical world)… or more Ornette Coleman and Albert Ayler in the Free Jazz world. And then there’s the super bizarre noise/performance-art music of now. It’s all music because people say it is.
It’s real real real difficult.
I love checking out all this stuff, but I can’t try and put it on around friends and family… or even say I enjoy it or get it.
Pretty awesome regardless.
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That!!!^^^
Here is another look at this (with the dorian b2; i.e., the second mode of the melodic minor scale) and even Beato calls it at some point “the key of A dorian b2” - so, there you have it.
The other take home messages from this video (my “aha” moments) are that: 1) yes, these modes can be the tonic of a song (or a melody, as I speculated further above); and 2) if you encounter any of the chords he mentioned and want to improvise, you can use the notes of the A dorian b2 scale 
So, it is often about knowing which scales to use to improvise on certain chords!
I know! It was a bit tongue-in-cheek what I said, but it is kind of sad how most music that does not adhere to the popular melodic framework gets easily pushed aside as “too far out there”. But, yeah, some stuff is really out there 
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Splitting the topic has to go to @JoshFossgreen.
This would make a great addition to the Theory category.
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Once more, very insightful.
I still haven’t made my peace with it, but you opened new angles for me to look at it.
And thank you for that, really! The information is priceless.
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Thannnnks.
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This might be another example of someone saying they’d like a glass of water and me blasting them with a fire hose… but here’s something else that might help(?)
Something that composers did to try and break new ground in composing in new keys/scales was use specific key signatures - so if you did want to write a piece in (for example) Dorian with a flat 2… you just plug those 7 notes into the key signature.
D dorian flat 2 would be a key signature of one flat - Eb. (Differentiating it from the F major key signature.)
This happens to share a key signature (as do all 7 note scale key signatures) with C melodic minor, Eb Lydian Augmented, F Lydian Dominant, G Mixolydian b6. A aeolian b5 and B altered.
So even when you customize a key signature to create a new palette of notes for your composition, no one knows what key center / root center / modal center you’re choosing from the available options until you make the music and they can see it on the page or hear it.
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@Gio Is there a reason this information isn’t specified or is it just expected you will know the theory. Like…
“Oh, yes. It was composed in D dorian flat 2 but follow along with what happens here and here. It’s resolving as Eb Lydian Augmented! Genius, I tell you! GENIUS!”
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Actually it helped!
So… we end right where we started? 
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Question. How many keys are there? Do you practice in all of them?
Hint…Victor Wooten talks about this…
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Close! See if you can find Victor talking about it. Very informative.
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Here we go!
Eric! This snuck past me.
This information is specified in the music if you’re reading or playing it. You don’t need to know it to talk about it, enjoy it or play it. Like ingredients in a delicious meal - you can still eat it, enjoy it, talk about it, etc. You could probably even cook it if you have a real discerning palette. But if you wanted to write the recipe, you’d have to get into the details and you’d need the correct language.
Curious, @kerushlow if you see/have a practical application for this information?
Personally? No, I’m not very advanced in theory. What is the difference between A# minor and Bb minor? Same notes from what I can tell. But there are things I know OF and don’t fully understand. Does it have to do with borrowed chords? What you would select as a key change? Which chromatic notes you might select?
Or is it of no significance? That’s possible too.
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The difference in a lot of the keys is subtle grammatical/spelling differences in the theory of how the music is written, and the language you use to describe the notes and chords in the piece.
A# minor is the relative minor of C# major, and has 7 sharps in the key signature.
Everything would resolve to an A# (needing a leading tone of G double sharp… yuck) and the chords would be identified from the key signature (all sharps).
Bb minor is the relative minor of Db major and has 5 flats in the key signature.
The leading tone is an A natural (much nicer than G double sharp) and all the chords would be identified inside of the 5 flat key signature.
It would sound exactly the same.
Why one instead of the other?
Depends on the instruments you’re writing for, what the composer feels about key signatures (Beethoven believed different keys communicated some feelings better than others), stuff like that.
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