Fully diminished seventh chord: A fully diminished seventh chord consists of four notes stacked in minor thirds, creating an even more dissonant and tense sound than the half-diminished seventh chord.
Minor key: A minor key is based on the natural minor scale, which has a different pattern of whole steps and half steps compared to major keys. It often conveys sadness or introspection.
Tritone substitution: Tritone substitution involves replacing one dominant seventh chord with another whose root is a tritone away. This technique creates harmonic interest and can lead to unexpected resolutions.