Many songs contain chords made up of more than three notes. By adding more notes on top of the basic triads, it is possible to create many other types of chords. The most common of these are seventh chords. By adding another note either a major or minor third above the basic triad, five different types of seventh chords can be created. Here are formulas for the various types of seventh chords.
The final type of seventh chord is the diminished seventh. This chord is unusual in that it contains a double flattened 7th degree ( ~7). This note is actually the same as the 6th degree (A) but it is technically called B~7 because the interval has to be some kind of seventh rather than a sixth because the chord is a type of seventh chord.
Here is the notation for the five types of Seventh chords built on the note C.
All these seventh chord types are used in Jazz and other music, particularly from the Romantic era onwards. By far the most commonly used is the Dominant 7th. In Blues and Jazz, a dominant 7th often ends a song, rather than resolving to a major or minor ending. To learn more about the various types of 7th chords and how they can be used, see Progressive Complete Learn to Play Keyboard Manual.