To understand what is happening with your C13 in the key of C Major requires a bit of understanding of the relationships between the scale degrees and the chords that are built on them
The Major Scale contains 7 primary notes (I refer to them as primary because all notes can be related to any Tonic which leads to chromaticism)
I use Roman Numerals: I II III IV V VI VII I
[In CMajor] C D E F G A B C
[In FMajor F G A Bb C D E F
If you were to stack 4 Cmajor scales on top of Each other in 3rds -- the first starting with C, the next E, then G, then B we form the primary 7th chords of C major
B C D E F G A B
G A B C D E F G
E F G A B C D E
C D E F G A B C
I II III IV V VI VII I
These are the naturally occuring 7th chords in a major Key.
I and IV is Maj 7
V is Dominant 7th
II, III, and VI is Minor 7th
VII is Half-diminished 7th (min7b5)
If you were to do the same process with the F scale, you would find the C is the V and has Bb as its 7th.
Does this mean that if you use a C7 (or any of its extensions) in the key of C major that you are not in C major (or worse yet that you are wrong?) NO!!! and NO!!!! You are using what is called a Temporary Dominant. Your C13 is borrowed from the key of F and creates a greater pull to the F chord (IV of C major). To truely establish F major as a new key (modulate to F), you would have to establish that Bb as a primary note instead of B nat. That takes more than a simple visit by Bb to throw the C major party out of whack.
In fact, you can at times use D7 (V of G), E7 (V of A) and A7(V of D) in the key of C major. A common progression is:
C E7 A7 D7 G7 C (Five foot two, Eyes of Blue comes to mind.) Each one is a "Temporary Dominant" of the next. This is just a variation of C Em7 Am7 Dm7 G7 C. You can even leave some as minor and change some to dominant and the Key of C Major will still have its say.
In the Case of C7 in Eb. It is the temporary dominant of the II (Fm). The root of C7 is the VI of Eb, so you can try Cm7 or C7 and see which works best. If the melody is Eb then you may need Cm7 or you might try C7 #9 (Voicing - E Bb D#(Eb)).
The other question that is implied is "Can you ever use C7 as I in key of C?" YES! The most common usage is in the basic Blues where the primary chords are all Dominant 7th -- C7 F7 and G7.
Also, there is music written in Mixolydian Mode ( Major Scale with lowered 7). The Beatles and early rock used it a bit. Usually, this music just used triads and although Bb major chord was common, the G7 (V of C) would retain its B nat.
In practice, you can actually relate just about any chord to any key. The keyness of the piece is not created so much by how much you stay in the 7 chords of that key, but by how each part relates to the overall key. Even if you were to truley establish and end up in another key for a time, if you work your way back, to the primary key it is as though you have never left. This is the principle behind Sonatas and Symphonies and Fugues. But that's a whole other story.
I hope that this helps.
This is the 1st time I've ever seen this
What I do notice is that the major chords from 1 to 7 are formed by stacking notes from the scale of C in different modes...
Does 1 always have to use the same scale in different modes?
or can I mix minor scales with major scales?
or Would I have to use scales that are related in the circle of 5ths or 4ths?
This would be very interesting to know....just when I thot I almost had the basic theory covered, all this erupts
Thanx in advance.
