By adding more notes on top of the basic triads, it is possible to create many other types of chords. After triads, the next most common chords are seventh chords. Various types of seventh chords are created by adding another note either a major or minor third above the basic triad. The formulas for the various types of seventh chords are shown below in the following diagrams. Like triads, once you have memorized each one, practice it in all inversions and transpose it to other keys.
The final type of seventh chord is the diminished seventh. This chord is unusual in that it contains a double flattened 7th degree (bb7). This note is actually the same as the 6th degree (A) but it is technically called Bbb7 because the interval has to be some kind of seventh rather than a sixth because the chord is a type of seventh chord.
The following example demonstrates all five 7th chord types in root position. Notice how changing one new note each time creates a new chord. Learn the formula for each chord type from memory and then build them on all twelve notes of the chromatic scale.