You already know that the distance between C and G is a 5th. But what kind of 5th? Not all 5ths are created equal. The distance from C up to G sounds different from the distance from C up to G♭.

To describe these differences, we give intervals a 'quality' in addition to their number. This quality describes their exact size and sonic character. The main qualities are Major, Minor, Perfect, Augmented, and Diminished.

The simplest way to find an interval's quality is to use the major scale as a measuring stick. When you measure the distance from the first note of a major scale, the tonic, to any other note within that scale, the interval will always be either Major or Perfect.

Let's use the C Major scale (C, D, E, F, G, A, B, C) as our reference.

Notice the pattern. The unison, 4th, 5th, and octave are 'Perfect'. They have a very stable, open sound. The 2nd, 3rd, 6th, and 7th are 'Major', which tend to sound brighter and more vibrant.

So what happens when a note isn't in the major scale? That's where the other qualities come in. We find them by shrinking or expanding the Major and Perfect intervals by one semitone, which is the smallest step in Western music (like from C to C♯).

Here are the rules:

• Minor: Lower a Major interval by one semitone. A Major 3rd (C-E) becomes a minor 3rd (C-E♭).
• Diminished: Lower a Perfect or minor interval by one semitone. A Perfect 5th (C-G) becomes a diminished 5th (C-G♭).
• Augmented: Raise a Major or Perfect interval by one semitone. A Perfect 5th (C-G) becomes an augmented 5th (C-G♯).

These alterations change the emotional feel of the interval. Minor intervals often sound darker or more melancholic than Major ones, while Augmented and Diminished intervals create tension and instability.

An interval of particular importance is the tritone. This is an interval spanning three whole tones, such as C to F♯. It can be written as an augmented 4th or a diminished 5th. In music history, its dissonant and unsettling sound led it to be called diabolus in musica—the devil in music.

What happens if you take an interval and flip it upside down? This is called inversion, and it follows some very neat rules. If you take the lower note and move it up an octave, you get the interval's inversion.

• Numbers always add to 9: A 3rd inverts to a 6th ($3+6=9$), a 2nd inverts to a 7th ($2+7=9$), and so on.
• Qualities change predictably: Major becomes minor, and Augmented becomes diminished. Perfect intervals, however, remain Perfect.

For example, the interval C up to E is a Major 3rd. If we invert it, we get E up to C. The number becomes a 6th ($9-3=6$), and the quality flips from Major to minor. So, E up to C is a minor 6th.

So far, we've only looked at intervals within a single octave. Intervals larger than an octave are called compound intervals. A Major 9th is simply a Major 2nd plus an octave. A Perfect 12th is a Perfect 5th plus an octave. To simplify them, you can subtract 7 from the number to find the simple equivalent (e.g., a 10th is a type of 3rd, since $10-7=3$).

Ready to test your knowledge?

Understanding interval qualities is the key to hearing how harmony works. These relationships are the building blocks for the chords and progressions we'll explore next.