7th Chords on Guitar: Major 7, Minor 7, Dominant 7
Seventh chords are ordinary major and minor chords with one extra note - the seventh degree of the scale. That one note changes the whole mood. There are three families, and each sounds completely different.
Dominant 7 (written as plain "7": E7, A7, G7)
Formula: 1 - 3 - 5 - ♭7. This is the bluesy, twanging chord. It sounds unfinished on purpose: it pulls hard toward the chord a fourth up. That pull is the engine of blues, folk, and rock and roll.
E7
A7
D7
B7
You already know one more: G7. Try E7 to A and hear the pull resolve. That is the most important move in blues.
Major 7 (maj7): the smooth one
Formula: 1 - 3 - 5 - 7. Dreamy and relaxed - the sound of soft pop, jazz, and Sunday mornings. If a chord sheet says Cmaj7, this is what it means.
Cmaj7
Amaj7
Dmaj7
Emaj7
Gmaj7
Compare C and Cmaj7 back to back: same chord, but the maj7 version floats. You also know Fmaj7 from the beginner chart.
Minor 7 (m7): the mellow one
Formula: 1 - ♭3 - 5 - ♭7. Softer and rounder than a plain minor. Em7 is arguably the easiest chord on guitar - two fingers and every string rings.
Am7
Em7
Dm7
How to practice them
Pick one family per day. Play the plain chord, then the 7th version, four slow strums each. Your ear will learn the color faster than your fingers learn the shape.