A 7th chord is a major or minor chord with one extra note: the flat 7th of the scale. That one note adds tension, motion, and (in folk and blues) a sound you instantly recognize as "old-time."
The shapes
G7: standard G, but instead of the ring finger on the high E at the 3rd fret, your index plays the 1st fret of the high E. Sounds like G but with a bluesy twist.
C7: standard C, but pinky on the 3rd fret of the G string (adds a Bb). Bluegrass and folk love this.
D7: easier than D for some. Index on the 1st fret B string, middle on the 2nd fret G, ring on the 2nd fret high E. Strum from the D string down.
Am7: standard Am, but lift the ring finger off the G string. Two fingers down: index on 1st-fret B, middle on 2nd-fret D. Wide-open, jangly sound.
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A longer write-up with every detail, drill, and common pitfall.
When to use them
7th chords resolve harder than plain major chords. They want to go somewhere.
The classic move: G → G7 → C. The G7 creates tension; C resolves it. Used in every Beatles song, every blues, every folk standard.
Try a 12-bar blues in E
A simple 12-bar blues in the key of E uses E7, A7, and B7. Play four bars of E7, two bars of A7, two of E7, one of B7, one of A7, and two of E7 to close.
You don't need barre chords for this. You don't need to add anything fancy. The 7th chords carry the entire feel.
Read the full guide
A longer write-up with every detail, drill, and common pitfall.
Next: suspended chords. Sus2 and sus4, the ringing acoustic flavor.