Tech question on 'diminisheds'

[nolly]

I'm new to pianoforum and am wondering if some bright person can help with a technical question please.

Why is it that diminished 7th broken chords as set for pianoforte practical examinations commence on the leading note and not on the tonic of that key?

Thanks in advance.

Nolly

[bernhard]

The diminished 7th is based on the tritone.

Consider C major. If you build a triad on each note of C major, the only tritone to be found in C major is BDF. (CEG is major, DFA is minor, EGB is minor, FAC is major, GBD is major. ACE is minor).

The same will of course be true of any major scale: the tritone can only be built from the leading note.

Therefore the diminished 7th on the scale of C major is Bdim7. And therefore (as you observed) the diminished 7 will always start on the leading note of the corresponding scale.

Likewise, in exams, they ask for the dominant seventh, which in the scale of C major is actually G7 (because you can only build a seventh chord on the scale of C if you start on the dominant G).

It is just a conspiracy to confuse the uninitiated

I hope this helps,
Bernhard

[surendipity]

The examiner will ask you to play A- diminished 7th solid or broken cords.
If you start on the Tonic of A  which will be (A) you will be playing the dimished 7th for Bflat-.


They usually ask you to Play the diminished 7th of a given key.

If so go down one semi tone and build from there.

If they ask you to play a diminished 7th starting on a given note, than do just that.  However it is rare.

They usually ask you to play the diminished seventh of a given key.