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I was told that the 7 in Cmaj7 indicates the 7th scale degree.

But here's what's confusing to me: in the C major scale, the 7th scale degree is B. And the interval from the root note of the Cmaj7 chord to the 7th would be a M7.

However, in the C Dorian scale, the 7th scale degree is A#, and the interval from the root note of Cmaj7 to the 7th would be a m7.

So if the number 7 in Cmaj7 really indicates the 7th scale degree, then does that mean Cmaj7 in C major scale is different than Cmaj7 in C Dorian scale?

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    Cmaj7 is Cmaj7. There is no Cmaj7 made up from the notes in C Dorian. There will be Bb maj7 and Eb maj7 instead. There is no A# note in C Dorian. It's Bb. – Tim Jul 25 '18 at 5:41
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C Ionian is C D E F G A B, whereas C Dorian is C D E♭ F G A B♭. (Note that this last pitch is not A♯, as you wrote.)

When you say that "the 7 in Cmaj7 indicates the 7th scale degree," I have one small adjustment: "the 7 in Cmaj7 indicates the 7th scale degree of the major scale" (hence the maj 7).

So in C Ionian (or C Major), the seventh chord built on C is Cmaj7, because the pitches are C E G B, taken straight from the C-Ionian collection.

But in C Dorian, the chord will be C E♭ G B♭, which creates a Cm7 chord.

In short, I think your confusion lies in the distinction between "Cmaj7" and "the seventh chord built on C." The quality of the seventh chord built on C will not always be maj7; instead, it will change depending on the current key signature.

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CMaj7 always has the notes C E G B regardless of the context around it. In general terms, any major 7th chord contains a root, Major 3rd, Perfect 5th, and Major 7th. It is naturally built in the C major scale by building a chord starting on C in 3rds, but it does not stop you from using it elsewhere even when the notes are not naturally found in the key/scale you are playing. You can play this in any key and it will contain the same notes.

You also seem to be confused with what C Dorian is. C Dorian not only does not have the B, but it also has no E. The notes of the C Dorian scale are as follows:

C D Eb F G A Bb C

If you naturally wanted to build a seventh chord off the root of the scale, it would be Cm7 which contains the notes C, E♭, G, and B♭ or in other terms contain a root, minor 3rd, Perfect 5th, and minor 7th.

Also note: there is no A♯ in C Dorian. The note is B♭ since if you spelled it as A♯ you would have a scale with two As and no Bs. For more information on how notes are named in different contexts, see this question.

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Your confusion might arise from a mistaken assumption that the 'maj' in Cmaj7 refers to the 'C'. Actually it refers to the '7'. That is to say, the 7th interval added to the C triad is a major 7th.

By convention, major triads have no suffix, minor triads have a 'm' suffix, and diminshed has 'dim' or '°'.

For the 7th interval the convention is sort of backwards. Minor 7th intervals are written just as '7' (minor implied, no prefix), and major 7th intervals are written as 'maj7', 'M7', or even just '△' as a shorthand.

Note the upper-case 'M'. If you see Cm7 then that is a C minor triad with a 7th, and because the 7th isn't prefixed with anything (the 'm' belongs to the C) it is a minor 7th interval.

Indeed, there is such a thing as a minor major-7th chord, e.g. C, E♭, G, B, sometimes notated as CmMaj7 although there are many other notations.

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    3rd para. True a '7' without a prefix has a minor 7th interval, but the chord produced will either be minor 7 OR dominant 7 – Tim Jul 25 '18 at 12:39
  • @Tim I'm not entirely sure what you're getting at. I don't think F7 in the key of C major could strictly be called the dominant 7th when it's actually built on the sub-dominant chord (F). If it was being used as a dominant chord then the implied key would be B♭. – Ian Goldby Jul 25 '18 at 14:28
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    F7 is a dominant 7th even if your intent is not to use it for its dominant function. The feature of a dominant 7th is the tritone between the Major 3rd and the minor 7th makeing it want to resolve the typical dominant-tonic way – Dom Jul 25 '18 at 14:32
  • F7 was not mentioned. All I am trying to do is clarify your 3rd para., which comes over (to me) as C7 = Cm7, which it doesn't. It could be phrased a little less ambiguously. Sticking with F7 - any dominant 7 will contain a maj. 3rd and a m7 (b7). whether it's based on a sub-dominant root is immaterial. – Tim Jul 25 '18 at 14:32
  • Thanks for the comments. I've edited my answer for clarity. – Ian Goldby Jul 25 '18 at 14:42

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