Intervals IV
The next two intervals we'll consider are the Perfect Fourth (5 semitones and the Tritone (6 semitones).
| Number of semitones | Abbreviation | Scale-degree / chord-tone | |
|---|---|---|---|
| Prime (unison) | 0 | P1 | 1 |
| Major Second | 2 | M2 | 2 |
| Minor third | 3 | m3 | ♭3 |
| Major third | 4 | M3 | 3 |
| Perfect fourth | 5 | P4 | 4 |
| Tritone | 6 | TT / +4 / °5 | ♯4 / ♭5 |
| Perfect fifth | 7 | P5 | 5 |
| Major Sixth | 9 | M6 | 6 |
| Minor Seventh | 10 | m7 | ♭7 |
| Octave | 12 | P8 or 8ve | 8 |
The Perfect Fourth
This is the complementary interval to the P5 (i.e. there is a P4 between 5 and 8) The chord movements in the earlier exercises have tended to involve the root ascending by a P4.1
We'll consider a single form for this interval:
The Tritone
The ‘Devil in music’, this is often considered to be the most discordant of the intervals.2 ‘Tritone’ is really a nickname.3 The interval is technically known as either an Augmented Fourth (+4) or a Diminished Fifth (°5).4 The +4 and °5 form a pair of complimentary intervals, which makes the tritone the only interval that is its own compliment. Truly diabolical.
We will only consider one tritone shape:
1
which is equivalent to descending by a P5 - when we talk about ‘root movement’ we're usually speaking in terms of pitch classes, not absolute pitches.
2
Unlike the semitone, the tritone's discordant nature is apparent even when the two notes are played sequentially.
3
Tritone = “three tones”, i.e. six semitones.
4
If you're curious, it should be called a +4 if the notes are four ‘letter-names’ apart, e.g. F-B (with F as the lower note) which outlines the sequence F-G-A-B and °5 if they are five letter-name apart, e.g. B to F (B-C-D-E-F). The term ‘tritone’ is an acceptable substitute for either.