interval
A perfect fourth
AlowerDupperP45 half steps
Five half steps. Turned upside down it becomes a perfect fifth.
On the keyboard
What is true of it
A up a perfect fourth is D: 5 half steps, and 4 letters counting both ends.
Turn it upside down — put the A on top — and it becomes a perfect fifth. The two numbers always add to nine, and the two sizes to twelve.
It is a perfect consonance.
Every interval on A
A → B♭minor second · dissonanceA → Bmajor second · dissonanceA → Cminor third · imperfect consonanceA → C♯major third · imperfect consonanceA → D♯augmented fourth · dissonanceA → E♭diminished fifth · dissonanceA → Eperfect fifth · perfect consonanceA → Fminor sixth · imperfect consonanceA → F♯major sixth · imperfect consonanceA → Gminor seventh · dissonanceA → G♯major seventh · dissonance