A seventh chord has four notes. It begins with the root, third, and fifth of a triad. The seventh is another third added to the stack odd numbered intervals. There are five types of seventh chords commonly used in tonal music of all kinds. They are defined by two factors; the type of triad and the quality of the seventh as follows:
Major triad with major seventh (Maj/Maj7)
Major triad with minor 7th (Maj/min7)
Minor triad with minor 7th (min/min7)
Diminished triad with minor 7th (dim/min7)
Diminished triad with diminished 7th (dim/dim7)
Refer to the Lead Sheet Symbols guide for how to notate the various sevenths. Play each of these on the piano or guitar, arpeggiating upward, to learn the quality of each aurally.