I'll keep this as short as possible hopefully i can get a nice short sweet answer.

Basically, say i come up with a nice chord progression like D F G C or something stupid like that. How would i know where to form a melody/solo in terms of scales. Obviously if the song is just the Cmaj chord all the way through so the Cmaj scale would work but when you have a chord progression how does it differ?

If the progression is in the key of C major, use the C major scale. The progression you posted is non-diatonic, but it still resolves firmly to C. Use the C major scale over the entire thing, and avoid the fourth over the D major chord (or raise it).
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ok thanks! is there a sort of logical method to working out what the key the progression is in?

I'm not strictly asking you to explain because it will probably take you ages i understand but if you know of any good lessons or tutorials that i should check out or any search terms that might lead me to an explanation that would be cool.

Thanks again
oh i think i understand... so if the scale you are using is made up of the notes in the chord then it should work?

I guess i should check out what the term diatonic means then as well

Diatonic means "in relation to the scale" more or less. To find the scale of a chord progression, you take all the notes of the chords and find out what scale they belong to.

C (C E G) D (D F# A) Emin (E G B) G (G B D)

This progression would be in the key of G.

G A B C D E F# G

As you can see, all the notes in chords fit into the G major scale, which is the best way to determine the key.

Also, the first and/or last chord in a sequence often indicates which scale is being used, as progressions often resolve to the tonic chord (chord built of the first scale degree).
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