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Diminished seventh

In music theory, a diminished seventh is an interval that has nine half steps (semitones). Explanations and excerpts from famous works

In music theory, a diminished seventh is an interval that has nine half steps (semitones ).

The a diminished seventh requires that:

The interval must be an seventh interval (seven note names between the first and the last).
The interval must have nine half steps.

What does a diminished seventh look like?

Here is an example of a melodic a diminished seventh (two music notes in a melody) and a harmonic a diminished seventh (in a chord):

How to recognize a a diminished seventh?

Rule of music theory: All intervals in a major scale starting with the tonic (degree I) are either major or perfect, and only unison, octave, fourth and fifth are perfect (the others are major).

Example with the interval B♭ / A♭♭:

Let's take the B flat major scale to have B♭ as the tonic:

From the rule stated above, the interval B♭ / A is major seventh, so the interval B♭ / A♭ is minor seventh, and so the interval B♭ / A♭♭ is diminished seventh.