In the Curious Community the other day we were discussing ‘demented’ 7ths. The husband of a Curious Piano teacher had referred to a diminished 7th in this way! It wasn’t the first time I had heard this however as several years ago one of my A level students always referred to a diminished 7th as a ‘demented’ 7th.
I think it’s rather a good alternative title and certainly helps bring the chord, with all its colour and vibrancy, to life. According to Andrew Higgins, one of our Curious Experts, ‘the diminished 7th chord is the most dynamic chord in tonal music’. 
Yet, knowing how the study and application of theory tends to drop off once students reach more advanced levels, I wonder whether your advanced students know this chord exists beyond the fact that it is needed for their next exam? I think it is about time pupils got better acquainted with diminished 7ths!
THE DIMINISHED 7TH
Diminished 7th chords are usually found in a minor key or passage. It belongs to the dominant family of chords and appears most often as vii°7 but can also be ii°7. It is made up out of a stack of minor 3rds and it is only possible to distinguish which note is the root of the chord from the way it is written.
Because the distance between each note is identical the chord has a very ambigious and unsettled feeling. Composers have exploited its great dramatic potential from the Baroque period onwards. It has often been used as a pivot chord and an opportunity to shift to an unexpected new key.
So let’s ‘mess around’ a bit with the diminished 7th. First as a standalone concept and then within the parameters of a Haydn Sonata movement.
MESSING AROUND WITH DIMINISHED 7THS
A diminished 7th can be formed on any note but in fact there are only three chords: Due to the fact that a diminished 7th is made up out of equally spaced minor 3rds there are only three different patterns:
#1 Messing AROUND; CHORDS
To start to explore its potential students have to be able to find their way around the notes. Why not play the dim 7 chord as a block in one hand whilst improvising with the other up and down using various rhythm patterns? Alternatively, play the broken chords using different mood words or imagery to stimulate the pupil: menacing, frightened, apprehensive, jittery, demented (!) for example. See the Musical Adjectives Project for more ideas.
#2 Messing AROUND; HARMONY
Experiment with harmonising a simple but well-known tune, for example Twinkle, Twinkle Little Star, just using diminshed 7th chords. Make sure you discuss with your pupil how this feels musically.
#3 Messing aROUND; SILENT MOVIE PIANIST
The diminished 7th was much loved by silent movie pianists. Find an old silent movie on Youtube, turn down the volume and improvise the soundtrack using diminished 7ths. If you want a bit more guidance on this Andrew Higgins provides more stimulation and ideas in his book So you want to learn to improvise?
#4 Messing AROUND with HAYDN
So let’s mess around with the Presto from Haydn’s Sonata in F major, Hob. XVI:23. It is currently on the ABRSM 2017-18 Grade 7 syllabus or can be downloaded from IMSLP.
I think Haydn is a highly underrated and underplayed composer. His music is full of treasures that are really worth exploring. According to Burkholder, Grout et al. Haydn’s music:
‘appeals at once to the least experienced listener yet rewards the connoisseurs, even after repeated hearings’. 
Haydn’s piano sonatas were written for amateurs and the sonata in question was written in 1773 for the man who would later become his patron, Nicholas Esterházy.
The F major sonata is typical of the classical style in more ways than we have time to explore fully here. Just think simplicity, balance, seriousness or humour according to the circumstances and you have some sense of the feel.
For example, look at how Haydn writes short gestures at the start, four of which combine together to make a longer phrase. The 3 mini-cadences punctuate the gestures building tension which is resolved with a perfect cadence in b. 8.
Charles Rosen argues that:
‘the emotional force of the classical style is clearly bound up with this contrast between dramatic tension and stability’. 
This is where the diminished 7 chord, with its shifting state and colourful feel, really comes into its own. See if you can find the first appearance of a dim 7 in the movement (answer at the bottom of the blog). When you play it how does it change the colour and mood of the passage?
Haydn of course is well known for his wit – just think about the Surprise or Farewell Symphonies. In the Presto we experience his humour in the dramatic development. He uses a diminished 7th chord, built on F# to suddenly shift the tonality from C major to G minor but for a couple of bars he fluctuates between a diminished 7th and the dominant 7th. The two chords are almost the same with just one significant exception: Eb becomes D.
Try playing this and listen to the way he teases us before finally settling into G minor!
So, did you find the first appearance of a diminished 7th in the movement? There is a brief, ‘teasing’ appearance in b.20 on the 2nd quaver before he moves on to fully establishing the key of C major.
 So you want to learn to improvise? Andrew Higgins, p. 9, 2015. Alfred Music UK
 The history of western music. Burkholder, Grout, Palisca, p. 535, 7th edition. 2006. W.W. Norton & Company
 The classical style. Charles Rosen. p. 74. 1971. Faber and Faber