A diminished fourth is an interval created by narrowing a perfect fourth, which, by definition, contains five semitones, by one semitone to four semitones. It is enharmonically equivalent to a major third because it spans the same number of semitones. Being diminished, it is deemed a dissonant interval. For example, the interval from C to F is a perfect fourth, spanning five semitones, while both the intervals from C sharp to F, and from C to F flat are diminished fourths, containing four semitones each.