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I'm learning about music theory and noticed that a C7 chord is based on the following notes: C E A# C E. I imagined a C7 chord should be based off the major scale where the 7th degree is the note B, thus making the chord: C E B C E. However, in reality it's using A#. What is the reasoning behind that?

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Can you tell us where you saw that? – Shevliaskovic Aug 20 '14 at 18:19
Note that this common fingering for C7 on the guitar is actually incomplete because it leaves out the 5th of the chord (the note G). What this means is that looking at ways chords are played on the guitar can be misleading if you want to analyze the structure of chords. – Matt L. Aug 20 '14 at 20:13
@MattL. The fifth is commonly omitted when voicing a dominant seventh chord, not just on guitar, but in general. A common voice-leading practice is to omit the 5th and double the root when voicing a dominant chord in a 4-voice texture, and it is almost always omitted in a 3-voice texture. – jshanley Aug 21 '14 at 3:43
A C7 chord is built from the C mixolydian scale, whereas a chord built from the C major scale is a Cmaj7. – Meaningful Username Aug 21 '14 at 8:18
@jshanley: I was not referring to common practice of voicing chords, but to the correct theoretical structure that a beginner should learn as a first step. It's kind of nice to know that there's a fifth in the chord, which you can - if you know what you're doing - leave out or replace it by the 13th etc. But that's a following step. – Matt L. Aug 21 '14 at 8:47
up vote 6 down vote accepted

The most correct notation for a C7 chord would be C E G Bb and not C E G A#. Note that while both Bb and A# are practically the same note, A is the 6th of C whereas B is the 7th of C. The notes that sound the same but are written different are called Enharmonic notes.

So, if you had a chord with these notes: C E A#, then that would be a C augmented sixth chord, written Caug6 or C+6.

Most common chords are build upon thirds. So,in order to create a C7 chord, you need:

  1. Root note (C)
  2. Major third (E)
  3. Perfect fifth (G)
  4. Minor Seventh (Bb)

If you want to create a 7th chord from the C major scale, with C as a root, then the result wouldn't be C7 but Cmaj7. That is because there is no Bb in the C major scale.

If you need some clarification between A# and Bb, you should check out this thread: Do I write # or b?

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C to A# is not a major 6th, but an augmented 6th. – Matt L. Aug 20 '14 at 20:08

But the C7 (or any 7 chord) is based off of the major scale. It's the V degree of the major scale. So for C major, the V is G7. This is because the V is the only degree that has a major 3rd but a minor 7th. The 7 chord is also known as a dominant chord (so the V degree is known as the dominant of the scale).

Based on the phrasing of your question ("the 7th degree is the note B") I think you're talking about C major in particular. When you play in C major, the first degree is a major 7th, or CM7. The second and third degrees are minor, so Dm7 and Em7. Then the 4th degree is major (FM7), then you have the dominant, G7, then another minor, Am7, and finally a minor with a diminished 5th, Bm7b5.

C7 is the 5th degree of F major, which as others have noted, has a Bb in it.

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-1: Seventh chords (not 7 chords) in general are not necessarily based off the major scale. "V degree of the major scale" - meaning what? (V, roman numeral for 5, is strictly used to refer to analytic chord functions, not for scale degrees.) Finally, the chord based on degree 7 of a major scale is called half diminished seventh. – ninemileskid Aug 21 '14 at 1:00

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