Diatonic intervals are a fundamental concept in music theory that describe the distance between two pitches in the degrees of a diatonic scale.
The term “diatonic scale” refers to the major scale or to the other six modes that use the same notes in a major scale. Since this system also refers to how music is written, we always have to consider spelling. But we always want to picture what happens on the piano keyboard.
Unison
Unison: 1 letter: there is only one pitch
Octave
No other name
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Half tone
Minor 2nd: 2 letter span: no key is skipped
Universal name: Half tone
C – B, F – E
Major 7th
Major 7th: 7 letter span: invert a minor 2nd
Universal name: Major 7 interval
B – C, E – F
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Tone
Major 2nd: also 2 letter span: 1 key is skipped
Universal name: Tone
C – D, D – E, F – G, A – B
Dominant 7 interval
Minor 7th: 7 letter span: Invert a major 2nd
Universal name: Dominant 7 interval
D – C, E – D, G – F, A – G, B – A
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Fat tone
Minor 3rd: 3 letter span: 2 keys are skipped
Universal name: Fat tone
D – F, E – G, A – C, B – D
3 Fat
Major 6th: 6 letter span: invert a minor 3rd
Universal name: 3 Fat
C – A, D – B, F – D, G – E
Bitone
Major 3rd: also 3 letter span: 3 keys are skipped
Universal name: Bitone
C – E, F – A, G – B
Quad
Minor 6th: 6 letter span: invert a major 3rd
Universal name: Quad
F – D, G – E, B – G
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4th
Perfect 4th: 4 letter span: invert a 4th
Universal name: 4th
C – F, D – G, E – A, G – C, A – D, B – E
5th
Perfect 5th: 5 letters span: invert a 4tth
Universal name: 5th
C – G, D – A, E – B, F – C, G – D, A – E
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Tritone
F – B, B – F
Augmented: 4 or 5 letter span: 4th or diminished 5th: 4 or 5 letter span
Universal name: Tritone