A seventh chord is a triad with a 7th added on top. It adds depth, tension, and a jazzy color to a chord.
The basic idea
The combination of the 3rd and the 7th changes the character, giving maj7, m7, 7, and so on.
What to check when using one
Replace part of a basic chord progression with 7th chords and check how they sit with the melody.
They are a more basic extension than tension chords, so learning 7ths first makes them easy to handle.
