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I do not have musical theory knowledge but I enjoy playing instruments being self-taught but now I am facing some limitations being non-theory musician.

I have a harmonica which is in the key of C, what kind of chords/scales I can play in guitar if I have such a harmonica key ?

Thanks

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    Do you want to play with a bluesy style? Commented Apr 7, 2022 at 14:26

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This answer assumes you have a 10-hole so-called blues harmonica.

A harmonica in the key of C has three chords: C, G and Dm.

If you play a song where you mainly intend to use the harmonica to play chords, the most suitable keys are C and G, and maybe (natural) Dm. This is since all these keys contain all the chords of the harmonica.

If you want to use the harmonica to play melodies or riffs without much bending you can use the notes from the three chords and the same keys are suitable, with the addition of Am (along with Em and F if you stick to pentatonic scales!) The additional scales are easier to play in the top two thirds of the harmonica, and for all keys except C and Am there are some "problematic" notes outside the scale that you need to avoid.

If you know (or are learning) bending and overblows more and more scales open up the better you get with these techniques, and with sufficient practice you can play songs in any key using your C harmonica!

One of the first scales people learn are the G minor pentatonic scale and the G blues scale, where the latter is really just the former with an additional note. As long as you stick to the lower half of the harmonica, these scales only require relatively easy bends, and they have a cool bluesy feel.

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On the assumption it's a diatonic harp, in key C, you can play anything in the key of C on guitar, and it will match perfectly.

However, if you wanted to play Blues on that same harp, you'd need to be in key G on guitar. That gives you, along with other things, the note F - which in Blues in key G is the ♭7, one of the prominent notes used in Blues. Another 'bent' note is B♭, which will be available as a draw note bent down from B♮ on the C harp.

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This page seems to give a nice description: https://www.harmonica.com/harmonica-keys-for-beginners/.

Basically, the matter is whether you are wanting a plain diatonic sound or a blues sound.

The plain diatonic sound could be for pop or folk songs. You would choose the harmonica key to be the same as the guitar key. That's straight forward.

Bluesy tonality is a bit more complicated, at least explaining the music theory behind it.

For a blues sound you would use cross harp where the harmonica key will be different (cross) from the guitar key. Blues tonality is especially defined by a plain major key with additional blue notes, notably a lowered seventh scale degree.

So, if you were playing blues in C, that means the seventh scale degree is B natural along with an additional, bluesy B♭. In terms of keys and key signatures we can thing of this as: change the B natural in C major to B♭ and we now have tones C D E F A G B♭, which is the key of F major. If the guitar player plays in C major and the harmonica player uses a F harp, the harmonica play can then play the bluey B♭ as well as most of the other tones of C major.

At some point you might hear someone say something about mixolydian mode and blues, that blues uses the sound of the mixolydian mode. Without going into the details of modes, let's just note that F major F G A B♭ C D E F is the same collection of tones as C mixolydian C D E F G A B♭ C. When you use an F harp it gives you tones that cover F major, but also tones for C mixolydian - a sort of bluesy C major, in other words blues in C.

For blues...

  • choose a harp a perfect fourth above the song/accompaniment key
  • choose an accompaniment key a perfect fourth below the harp key.

I have a harmonica which is in the key of C, what kind of chords/scales I can play in guitar if I have such a harmonica key ?

You guitar is the accompaniment.

You have a C harp.

For diatonic tonality, play the guitar accompaniment in C.

For blues choose an accompaniment key a perfect fourth below the harp key. A perfect fourth below C is G, so play the guitar accompaniment in G major. (The F natural from the C harp will provide the bluesy lowered seventh degree for the key of G major played by the guitar accompaniment.)

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