The 7th Chords.
Building from the triad (1-3-5) we can add another 3rd, that would give us the 7, (7th note in the scale).
Adding this 7th note gives us the “7th chord”.
There are 2 types of 7th chords: Major 7th and Minor 7th. (NOT to be mistaken with being a major or minor CHORD, ONLY the 3rd note determines that!)
Major and Minor 7th has to do with the DISTANCE from the 1, Minor 7th is 11 semitones away from the 1, Major 7th 12 semitones away from the 1.
The 7th chords have certain function, they have to do with movement.
We will only look at the MINOR 7th chords for now.
The Minor 7th chords have a certain tension in them, wanting to move to a “solution”.
If they move in an “authentic” way they will move UP a PERFECT 4th (5 semitones up, and always count the first note as 1)
In our example the chord G, added with a minor 7 (the note F), will want to move up a perfect 4th and land on C.
We call this dominant behaviour, the minor 7th chord is a dominant chord, in this example called G7.
G7 goes to C (4 up).
This is NOT a rule, as said, it would be an authentic progression if it does move up 4, but often we see it moves to somewhere else, like in Rock music.
In Jazz you will find many authentic progressions on which we can anticipate a lot.