Diminished fourth

In classical music from Western culture, a diminished fourth (Playⓘ) is an interval produced by narrowing a perfect fourth by a chromatic semitone.

[1][3] For example, the interval from C to F is a perfect fourth, five semitones wide, and both the intervals from C♯ to F, and from C to F♭ are diminished fourths, spanning four semitones.

[4] A diminished fourth is enharmonically equivalent to a major third; that is, it spans the same number of semitones, and they are physically the same pitch in twelve-tone equal temperament.

For example, B–D♯ is a major third; but if the same pitches are spelled B and E♭, as occurs in the C harmonic minor scale, the interval is instead a diminished fourth.

The 32:25 just diminished fourth arises in the C harmonic minor scale between B and E♭.