Why does the Dorian mode on C have two flats?

Question

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?

Answer 1

'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. :-)

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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

Answer 2

The dorian scale ain't a major scale. It's key is D and it has all white keys (in it's usual form), but that makes it NOT 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.

Answer 3

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.

Answer 4

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.