I'm studying scale modes and I just found on Wikipedia that the Dorian mode on C has two flats...
Why is this? Because the Dorian mode is just the major scale (Ionian) starting from the second note, shouldn't it keep the key signature? In the case of C, shouldn't it be: D E F G A B C ?
What am I missing?
'Dorian mode on C' does not mean "the Dorian scale that you can find among the notes that are available in the major key of C"!
'Dorian mode on C' refers to the Dorian scale, or set of note intervals, that start on the note C, i.e. C is its root or tonic. This set of notes happens to be the same as the ones found in the Bb major key, thus two flats.
This is to say, the specific mode gets its name (C, Db, D, ...) from its first -- or root -- note, not from the key who's notes happen to match the set of notes used in this mode.
Again:
The Dorian mode on C happens to use the same notes as the Bb major key and thus uses the same key signature. However since C is its root note, or tonic, it is a C Dorian.
This is analogical to that the Mixolydian mode on F uses the same notes as are found in the key of Bb but has F as its root, or to that the Ionian mode on Bb, which has Bb as its root, happens to use the same notes as the Bb major key, etc.
(Or for that matter that the Bb major scale happens to use the notes of the Bb major key. But this is slightly different since this scale got its name from the key. :-)
Perhaps these three tables clarifies this for someone:
1) All Dorian modes
Mode Flats/Sharps Corresponding major key signature C Dorian 2b Bb C# Dorian 5# B D Dorian 0 C Eb Dorian 5b Db E Dorian 2# D F Dorian 3b Eb F# Dorian 4# E G Dorian 1b F G# Dorian 6# F# A Dorian 1# G Bb Dorian 4b Ab B Dorian 3# A
2) All modern modes corresponding to the Bb major key
(Note that the number of flats remain the same.)
Mode Flats/Sharps Corresponding major key signature Bb Ionian 2b Bb C Dorian 2b Bb D Phrygian 2b Bb Eb Lydian 2b Bb F Mixolydian 2b Bb G Aeolian 2b Bb A Locrian 2b Bb
3) All modern modes on C
(Note that the number of flats/sharps varies.)
Mode Flats/Sharps Corresponding major key signature C Ionian 0 C C Dorian 2b Bb C Phrygian 4b Ab C Lydian 1# G C Mixolydian 1b F C Aeolian 3b Eb C Locrian 5b Db
By way of analogy: Isn't the minor scale just some other key's major scale starting on the sixth note? C minor has three flats, and this because C minor is just Eb Major, starting on the sixth note (that is, the sixth note of Eb Major, which is C). And A minor has no sharps or flats because it is just C Major, starting of the sixth note (of C Major, which is A).
Ok, well, in this same sense, the Dorian scale is some other key's major scale, starting on the second note---in the case of C Dorian, the "other" major scale is Bb Major. Hence two flats.
Think of it like this: go down a full step from C and figure out the key signature for the major key that corresponds to that note. A full step down from C is "Bb." The key of Bb major has two flats. Easy as pie.
I'm studying music myself, but I think I've learned enough to answer your question, just in case it hasn't been cleared up for you. Okay, to simplify it as best as I can…although the Dorian mode is built on the second letter of the C major scale(D), it can begin on any note. Basically, you should only use the D Dorian as a reference to show you whole/half step formula. The most important thing you need to know is the formula for it, which is W-H-W-W-W-H-W. So, whatever note you start on, write down the following notes that correspond to the formula's step and you will notice that some notes will be flatted or sharped(or flattened or sharpened). Let's do the formula steps in the key of C. C-D=whole D-Eb=half Eb-F=whole F-G=whole G-A=whole A-Bb=half Bb-C=whole
And remember whatever key you're using in the Dorian mode, you call it by that key followed by the word Dorian. In this case, you would call it C Dorian.
It all started with a lyre. If a lyre was tuned in a major scale, the ancient Greeks noted that you could also play a melody starting from a point under the root of the major scale. So if you called that major mode Lydian, you could start from the string three strings higher - physically higher (nete), not higher on a staff, which wouldn't be invented for a few thousand years - and get an entirely different affect. This was the hypo (under) mode. So that one would be called HypoLydian. Then they also noted that you could tune one of the strings down (the one physically up from the root) and get the same affect, but with the root in a different place. Now, if you did that, you would have a hypo for that mode too, and you could call it HypoHypoLydian but that's getting really weird, so they came up with a different scheme which utilized four basic modes, each with its own hypo phase.
Then the Renaissance came, and scholars attempted to recreate the Greek modes based upon one codex which was something like Mel Bay's "How To Tune Your Lyre". They got it all wrong, which is to be expected. I love that they thought "nete" (higher) meant what we think of as a higher note, not that it was a physically higher note on a lyre. A lyre is strung like a guitar, with the "higher" note physically lower. So the church modes actually go the opposite direction from the Greek modes.
But hey, they got opera all wrong, and we've got one helluva art form to show for it.
In response to Stephen's answer, the Dorian scale ain't a scale at all!! It's a MODE. I guess the two scales that ARE modes are major (Ionian) and aeolian (natural minor). Each and every mode has to belong to,and have the same notes as, a major scale.Thus C Dorian,starting on the second note of Bb, will have 2 flats as its signature, The Dorian of C itself will have an open signature, as it 'starts' on D. Confusion reigns with C Dorian and Dorian of C. The key signature only tells you which notes are in use in a piece ; you need to check where 'home' is - often the last note of the piece, or the first note in the first full bar. This will usually tell what mode (or key) it's in.
The dorian scale ain't a major scale. Its key is D and it has all white keys (in it's usual form), but that DOESN'T make it a major scale. Dorian steps are whwwwhw. Major is wwhwwwh. If you transpose a Dorian scale from d down to c, you need to preserve the "dorian scale of whwwwhw". Hence the flats.
D E F G A B CDorian mode on D. The word "Dorian" tells you where the half and whole-note steps are, the C or D tells you where it starts from. It doesn't mean that it's based on C or D major, just like C minor is not using the same notes as C major.