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I started studying music theory on my own but could use someone to confirm if i'm correct

I'm assuming that the progression D - Cadd9 - G is in the key of D major.

The D major scale contains the notes: D, E, F♯, G, A, B, and C♯

But when i'm playing over the D major scale on my guitar, the C# note never seems to sounds right.

I know that Cadd9 contains these notes: C - E - G - D (there's no C# in there)

The G major chord also does not have a C#: G - B - D

And the D major chord neither: D - F# - A

So if i understand correctly, It is not because a note is in the major scale of a certain key that it always can be played, it depends on the chord played at that moment right? For example, will a G note sound right over the D chord of the progression?

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    If you can make it feel like it modulates between the keys of D major and G major, then C# will be OK during those moments when it's in the key of D major. Add a fourth chord and make it: D, C, G, A. During the A and D chords, play D major scale and emphasize the C# note. During C and G chords, play G major scale and emphasize the C note. Voilà, it changes keys. – piiperi Jun 18 at 23:03
  • Thanks for the advice – DennisVA Jun 18 at 23:10
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    C# can be made to fit, but for the purposes here, it doesn't appear to for the reason that it's actually not a diatonic note.The key is much more likely to be G major - or a mode thereof - which contains C nat. not C#. – Tim Jun 19 at 6:44
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    @Tim It's not a diatonic note if the key feels to be G, tonic G. It's good to learn to feel the tonic in addition to logically deducing it using sets of rules. It's such an elementary thing to feel where the tonic is, because understanding of many more complex things build on that sensibility later on. Actually the whole question arose from not being able to feel the tonic, but instead relying on a rule, "first chord is tonic". I thought the modulating progression D - C - G - A might help towards learning to feel the tonic, or at least learning about the concept of "feeling the tonic". – piiperi Jun 19 at 7:04
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    @piiperi - wasn't aware of the 'rule' that 1st chord = tonic! Just playing those 3 chords points emphatically towards G, with a plagal cadence. OP must have felt that C# was out of place - otherwise the question wouldn't be here. The 'rule' that diatonic notes tend to fit best isn't a bad one. Certainly better than 1st chord = key. Far better to try the 'rule' last chord = key. – Tim Jun 19 at 7:10
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I'm assuming that the progression D - Cadd9 - G is in the key of D major.

Why are you making that assumption? Those 3 chords contain the notes of a G major scale, so it would make much more sense to assume that the key is G major.

EDIT: Elaboration. You can play the note C♯ on top a D major chord in case you want that particular sound. But since your assumption of the key being D major is based on nothing but the 3 chords and that doesn't call for such an assumption there is no C♯. Try out playing the G major scale instead and listen to how that fits.

  • I was assuming that because it's the first chord, but you're right. Is it in G major or are you just pointing out that i could be wrong? – DennisVA Jun 18 at 22:51
  • Wow this is a real eye opener, it was stupid to just take the first chord and assume that it was the key! Thanks alot! – DennisVA Jun 18 at 22:56
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    @DennisVA You are welcome. I edited the post to make it more an answer type of thing which is of better quality and I can see you have accepted the answer. – Lars Peter Schultz Jun 18 at 23:05
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One more answer.

A C# note can be played perfectly well in a song that has the chord progression D - Cadd9 - G. The key or mode is what you make of it, within some limits, but the biggest limit might be your creativity. If you play or sing a melody like it's in the key of D, then it's in the key of D. If you play or sing a melody like it's in the key of G, then it's in the key of G. If you play like D mixolydian, then that's your feeling. If you play blues scales and minor pentatonics on the chords, than it's different again - and you can change your mind in the middle of the progression, if you want. The chords can lend themselves to many different things. The thing you cannot make to sound credible very easily is to play the C# note on top of the Cadd9 chord, because C# is right between the C and its add9 i.e. D notes. (Short passing tones don't count)

Here's a tounge-in-cheek example that plays C# notes very loud and clear on the chords, and I don't hear any problem with it. Then it changes to something else whenever the C chord comes. In the final repetition, there's D minor pentatonics or bluesy D mixolydian. The backing chords are the same every time. https://vocaroo.com/i/s1kXeFcqpGwW

Which one feels more like a "true" tonic - D or G? Is the feeling the same all the time, or does it change along the way?

  • This is by far the best answer! Where did this audio come from btw? – coconochao Jun 19 at 21:40
  • @coconochao I made it, because I felt that purely textual explanations don't really cut it. It's not the best possible example, because my solo only shows a couple different variations out of the many possibilities. The first chord is played twice as long as the others, and the melody lines land a certain way, which affects the feeling and steers interpretation perhaps too simplistically. Anyway, I'm just trying to give a different viewpoint to complement the other answers. – piiperi Jun 19 at 22:06
  • Brilliant!! Your answer couldn't be more complete!! Great solo btw! – coconochao Jun 19 at 22:12
  • This is why we need an audio player plug-in on this site. – Your Uncle Bob Jun 20 at 0:19
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    @DennisVA if you want it to sound like it's in D major, try to trick yourself into believing it's in D. (1) Start with a proper D major cadence as a lead-in: D - G - A7 - D to get in the mood. (2) Take this modified chord progression: Dmaj7 - D7 - G - G/A, (3) play or write your melody on top of that modified progression, (4) keep the melody, but change the underlying chords: replace Dmaj7 with D, and D7 with Cadd9, and G/A with just G, so it becomes your original progression. (And throw away the cadence). Does this work for you? The key is whatever you can make yourself believe it to be. :) – piiperi Jun 20 at 8:02
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You are correct in saying that the above progression is in D major. However, it is not written using the typical D major scale. Instead you are using a version (mode) of the major scale called mixolydian.

The notes in D major are:

D E F♯ G A B C♯ D

D mixolydian contains the notes:

D E F♯ G A B C D

D mixolydian is a mode of the G major scale meaning that they have the same notes.

I think your getting mixed up because a scale and a key are not the same thing.

A scale is a collection of notes that you refer to by using a starting note and a pattern, e.g., D (starting note) major (pattern). The pattern in this case is (W = whole step, H = half step): W W H W W W H

Whereas, a key, while it is linked to a scale (major key → major scale), is more about the how the chords and notes are used. You can stay in a key while using chords and notes that are not in the scale, e.g., modal interchange.

I'm guessing your progression is in D mixolydian is because it is a I - ♭VII - IV and that is one of the most popular progressions using mixolydian. It could be said to be in G major, but in that case it would be a V - IV - I which is less popular. (There is a similar progression, a I - V - IV - I that is extremely popular.) Also, D, Cadd9, and G all contain a D, which makes it all the more likely that D is tonicized.

  • If the progression is indeed in D Mixolydian, you would call it that and not D major; but you really can't tell from such a small snippet of chords out of context. Just as reasonable to assume G major, also noting that it is very common for a progression to end on the tonic of the key. – ex nihilo Jun 19 at 1:43
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    Most people would say D mixolydian (including me), but actually D mixolydian isn't a key. Keys can only be major or minor. Songs using the D mixolydian mode are in fact in D major. You are right that quite often chord progressions on a I. They also often end on a cadence (e.g., four chord and 50s progressions). It could be in G major, but my guess is it's in D major. – Phoenix Jun 19 at 4:12
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    Progressions are in keys not in scales or modes. For example, C-G-a-F is in the key of C not in the C scale or in the Ionian mode. Keys have chords, scales and modes don't. – Phoenix Jun 19 at 5:24
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    Yea, it's confusing but technically true. Scales can make chords; they don't have chords. Scales are just collections of notes. Modes tell you which note to start on. Only keys have chords. Tbh, a lot of music terminology is a big, "well, yes but actually no." – Phoenix Jun 19 at 6:08
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    Sorry, but what you state is wrong. D major is a key, D Mixolydian is a mode. And D Mixolydian isn't the same as D major. In fact, it contains every single note of G major! D Mixolydian essentially uses the G major notes but centres or homes in on the note D instead of the note G. And - C G a F is in the key of C major, and uses the C Ionian mode notes. This answer needs a little tlc (editing), as there are several inaccuracies involved. – Tim Jun 19 at 6:39

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