It's extremely dissonant if the two notes are only a semitone apart, but if you substitute an E an octave up, you get an interval (11 semitones in equal tempera

It's extremely dissonant if the two notes are only a semitone apart, but if you substitute an E an octave up, you get an interval (11 semitones in equal temperament or a 15/8 interval in just intonation) that appears in a major7 chord.