Chords are notes played simultaneously. The most commonly used chords are constructed from stacked thirds. Chords can also be constructed from seconds, fourths or fifths but these types of chords are less common.
Seventh Chords:If we stack another third onto any of the basic chords we now have Seventh Chords.
From left to right the names of these chords are: Major-Major seventh, Major-minor seventh, minor-minor seventh, minor-Major seventh.
These chords names are commonly abbreviated, such as M7 for Major-Major Seventh and 7 for Major-minor seventh.
From left to right the names of these chords are: Augmented-Major seventh, Augmented-minor seventh, Half diminished, diminished seventh.
Learn about Extended Chords
Learn about Chord Symbols