Diminished 7th chord

Diminished 7th Chord

A Diminished 7th Chord is similar to a Dominant 7th chord in two ways: It is made up of 4 notes, and it contains a 7th (a diminished one, which is actually a major 6th - mind blown).

That is where the similarity ends, because they don't sound the same, and they are not built in the same way.

The way we build a Diminished 7th chord is as follows:

Simple version:

Step 1: Get a triad (root, 3rd, 5th)
Step 2: squeeze the 3rd and 5th down a semitone each (root, 2ndandahalf, 4thandahalf)
Step 3: stick a 6th on the top.

I know, right? Not the most logical order of events.

The actual "pattern" for the mathematically minded people among us is Root, ♭3, ♭5, ♭♭7 (aka a major 6th)

So to build a C diminished 7th chord you would do this:

Degree of chord/scale		Application of ♭’s?	Resulting note	Enharmonic equivalent
Root	C	None required	C	C
3rd	E	Add one ♭	E♭	D#
5th	G	Add one ♭	G♭	F#
7th	B	Add two ♭’s	B♭♭	A

And here's how it would look on a stave:

There's ways of describing chords when they get interesting like this:

In Popular Music, it would be expressed as Cdim7 but there is a shorter version: C^o7

This is the way many Classical musicians studying harmony or using symbol shorthand to compose will opt for. If you get to Uni and see that little "o" in a chord context, it stands for Diminished.