Theory Thursday: Fourths

Intervals: The Fourth

In the last couple weeks we learned about 2nd and 3rd intervals, and their major and minor varieties. Today, we're moving right along to the next interval, the fourth.

So from last week, we learned that thirds are the intervals created by skipping over a tone in the scale. This week, we're going to talk about the interval relationship that happens if you skip two scale tones. Using the A minor scale, here's a look at the 4th intervals:

A -> D
B -> E
C -> F
D -> G
E -> A
F -> B
G -> C

None of that seems too bad so far, right? It's really a lot like everything else we've covered so far, it's just that the interval is getting bigger. So to build a fourth, if we start on A, we simply leap-frog over the B and C to get to D, and then we do the same for every tone in the A minor scale.

Let's now take another look back at our pattern of whole and half steps that make up our A minor scale (I included the scale on top and the interval between each tone below...):

A B C D E F G A B C etc
 W H W W H W W W H  etc

Again, remember, every W is 2 half-steps. So let's now take a look at the raw intervals that make up each fourth interval in the A minor scale. We do this by adding all the intervals between our two tones. So for example, A to D is W + H + W (using the pattern above), the B to E is H + W + W, C to F is W + W + H, etc. So this looks like:

A -> D : W+H+W = 5 half-steps
B -> E : H+W+W = 5 half-steps
C -> F : W+W+H = 5 half-steps
D -> G : W+H+W = 5 half-steps
E -> A : H+W+W = 5 half-steps
F -> B : W+W+W = 6 half-steps
G -> C : W+W+H = 5 half-steps

Remember how there are major and minor seconds and thirds? Well things change a little for fourths. As you can see from the diagram above, most of the fourths are made up of 5 half-steps, and just one of them is 6 half-steps. Fourths that consist of 5 half-steps are called "perfect" fourths (P4) and because the fourth made up of 6 half-steps is just a little bigger, it's called an "augmented" fourth (A4).

So the important things to remember about fourths are 1) a fourth is the interval that results from skipping 2 scale tones, and 2) a perfect 4th (P4) is 5 half-steps, and 3) an augmented 4th (A4) is 6 half-steps.

Review

• How many half-steps in a minor 2nd interval?
• How many half-steps in a major 2nd interval?
• How many half-steps in a minor 3rd interval?
• How many half-steps in a major 3rd interval?
• How many half-steps in a perfect 4th interval?
• How many half-steps in an augmented 4th interval?

Reminder

If you have ANY questions or comments, please leave a comment here, on my Facebook page, or tweet at me (@chrisstarkgtr).