A half-diminished seventh chord is a four-note chord consisting of a root, minor third, diminished fifth, and minor seventh. It is often used in jazz and has a melancholic and mysterious sound.
Fully-diminished seventh chord (°7, dd, or diminished-diminished): This chord consists of a root, minor third, diminished fifth, and diminished seventh. It has an even more dissonant and tense sound than the half-diminished seventh chord.
Dominant seventh chord: This four-note chord consists of a root, major third, perfect fifth, and minor seventh. It has a strong and resolved sound commonly found in blues and rock music.
Minor triad: A three-note chord consisting of a root, minor third, and perfect fifth. It has a sad or melancholic quality often associated with classical music.