interval
A diminished fifth
AlowerE♭upperd56 half steps
A perfect fifth narrowed by a half step. The interval inside every diminished triad.
On the keyboard
What is true of it
A up a diminished fifth is E♭: 6 half steps, and 5 letters counting both ends.
Turn it upside down — put the A on top — and it becomes a augmented fourth. The two numbers always add to nine, and the two sizes to twelve.
It is a dissonance.
Six half steps can be written two ways: A to D♯ is a fourth, A to E♭ is a fifth. They sound the same and behave differently.
Every interval on A
A → B♭minor second · dissonanceA → Bmajor second · dissonanceA → Cminor third · imperfect consonanceA → C♯major third · imperfect consonanceA → Dperfect fourth · perfect consonanceA → D♯augmented fourth · dissonanceA → Eperfect fifth · perfect consonanceA → Fminor sixth · imperfect consonanceA → F♯major sixth · imperfect consonanceA → Gminor seventh · dissonanceA → G♯major seventh · dissonance