For each key center, we are going deeper, all while scratching the surface in new keys [triads, then 7ths, then more!].
8 to the 7: 7th chords can be created by taking the highest root [octave – 8] in the chord and dropping it back to the next chord tone in the scale. It will be either 1 or 2 frets back.
7 for each chord
7 = Maj7 = Major 7 = 7th that is 11 half steps away from root.
7 = 7th is 10 half steps away from root.
If the 7 is in a Major chord, it is a Dominant 7 [just 7].
If the 7 is in a minor chord, it is a minor 7 [m7 or min7 or –7].
m7♭5 = minor 7, flat 5 = half diminished 7 
Once 7’s the chords are I IV are Major 7, V is Dominant 7, ii iii and vi are minor 7, viio becomes m7♭5.