One of the most useful chords of all is the diminished 7th chord, often notated as “dim7” as in “Cdim7”, “Dbdim7”, “F#dim7” and so on. They can be used for so many different things, from transitions to transposing to modulation to increasing the emotional intensity of a song. If you listen to “Moonlight Sonata” you will find an entire section of the piece where Beethoven toggled back and forth between a diminished 7th chord and a minor triad, then back to a dim7, then a minor, etc, etc. Watch this short video on dim7th chords and you’ll catch on quickly: