Perfect Fourth and Augmented Fourth

Perfect Fourth: Named for the fourth note of the major scale. It’s an interval of five semitones. It’s called “perfect” because, compared with the augmented fourth, it sounds a lot more, um ... “perfect.” At least in the context of a chord or a tune.
Augmented Fourth: A wild, unruly interval, it’s also named for the fourth note of the major scale. However, the augmented fourth overshoots the perfect fourth by a semitone, for a total of six semitones. This interval has several other names. It’s often called the tritone because it spans three whole tones (six semitones). It’s also known as the diminished fifth, because it’s a half-tone short of being a “perfect” fifth. In the Middle Ages, they called it diabolus in musica—the “devil in music.” Somebody had a sense of humour way back then. Or ... maybe they believed it was the musical devil hisself.