Next step in chord progressions
The flat two, seventh chord. IIb7
C Major scale, 2nd note D down ½ tone Db add a 7th = IIb7
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The V7- I progression (G7- C) becomes
IIm7- V7 - I progression. Dm7 G7 C

Add the IIb7 (Db7) -- IIm7 – V7 – IIb7 – I Dm7 – G7 – Db7 – C You can also drop the G7 --- IIm7 – IIIb7 – C
Good for endings and intro, and a lot more.

ADDING EXTENTIONS TO A MAJOR CHORD.

C major scale
C D E F G A B C D E F G
Major is built with thirds—C to E -- E to G
I III V of the C major scale

C6- CEGA Cmaj7th- CEGB Cmaj9th- CEGBD
Cmaj11th – CEGADF Cmaj13th – CEGADFA Careful with the Cmaj11th

And no, you do not add all the notes to your C major chord, just the ones that sound good to you.

Stay with me, this is going to get very interesting
John C.