 The augmented fourth ("A4") and diminished fifth ("d5") are defined as the intervals produced by widening the perfect fourth and narrowing the perfect fifth by one chromatic semitone.They both span six semitones, and they are the inverse of each other, meaning that their sum is exactly equal to one perfect octave (A4 + d5 = P8).In twelve-tone equal temperament, the most commonly used tuning system, the A4 is equivalent to a d5, as both have the size of exactly half an octave.