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I'm relatively new to music theory, I was under the impression Cmaj7 was the same as C7 so i just added a minor third above the C triad (CEG) and got CEGBb. As it turns out, C maj7 is actually a major third above the C triad - that is CEGB.
Then there's the Cminor variation, that is C Eb G, and Cminor7 C Eb G Bb. If there is a discrepancy within the chords and the 7th why is there no discrepancy between regular C and C major?