Adding 7ths to Major Scale Chords

You can play new chords by adding 7ths to major scale chords.
It's a good place to start because it is in keeping with 3rds.
ie 1-3-5-7. Root, 3rd, 5th and 7th. So we now have a stack of 3rds.
You can easily add a 7th without suspending one of the other 3rds.
 
Using the G major scale and it's basic TRIAD to start, you add a
7th by counting up to 7 in the degrees of the scale. 
G A B C D E F# G
1 2 3 4 5 6 7  8/1

1   3   5   7
G   B   D   F# = Gmaj7 or GM7

There are two types of 7ths intervals.
1. The Major 7th, almost an octave, missing by just a one half step
   or one fret on the guitar,
2. The minor or b7th. It misses the octave by a whole step (2 half steps)
   Named by the type of 7th that occurs in a minor scale.

You see 7ths minor 7ths on the scales below.

G: 1-3-5-7  G-B-D-F#  Gmaj7 or GM7
A: 1-b3-5-b7 A-C-E-G  Aminor7 or Am7
B: 1-b3-5-b7 B-D-F#-A Bminor7 or Bm7
C: 1-3-5-7 C-E-G-B Cmajor7 or CM7
D: 1-3-5-b7 D-F#-A-C Dominant7 or D7
E: 1-b3-5-b7 E-B-G-D Eminor7 or Em7
F#:1-b3-b5-b7 F#-A-G-E F#minor7b5 or F#m7b5

The three major chords--G-C-D, in the Key of G (G major Scale)
occur on the 1st, 4th, and 5th degrees of the scale.
These are numbered with Roman Numerals, I IV V.
They don't have similar major 7th chords. G and C have a major 7th
but D has a flat or minor 7th.

A MAJOR TRIAD with C minor 7th is technically called a dominant 7th.
But, & this gets confusing, it's simply referred to as a 7th. You always have to specify Major 7th chords as Maj7 but you can simply use 7 to label dom7ths.

If a 7 by itself follows a letter name, then the chord is dominant.
(A Major Triad with a minor 7th) So in the G Major scale, the three 
Major Chords with 7ths become GMaj7  CMaj7 and D7.
The dominant 7th chord naturally occurs on chord V in the Major scale.

flyinglibra51
God Bless Glen Travis Campbell