Music Theory: Intro to Intervals

An interval is the measurement of distance between any two named notes. They are calculated by counting the lowest note as 1. Before calculating intervals, I’m going to first re-arrange the C Major Scale graphic to look more like a piano.

From…

to…

The 2nd Interval

A “2nd” interval is an interval that spans 2 note names. For example, C–D is a 2nd interval. C (1) and D (2).

Simply count just the named notes in the circle:

C–D is also an example of a whole step. Notice that you skip over the unnamed black note in between C and D. E–F is also a 2nd because it spans two names.

Again, simply count just the named notes in the circle:

E–F is also an example of a half step.

To figure out an interval just start counting from 1 and only count the letters. Counting intervals is foundational for when we get to creating power chords (and other types of chords).

The 3rd Interval

A 3rd is an interval that spans 3 note names. For example, D–F is a 3rd interval because 3 notes in the scale are spanned: D (1), E (2), and F (3).

Simply count just the named notes in the circle:

G–B

G–B is a 3rd because it spans 3 notes in the scale: G (1), A (2), and B (3).

B–D is a 3rd

To count from B to D, wrap around to the beginning of the scale after you have reached C but don’t count C twice. B (1), C (2), D (3).

So B–D is not counted as B (1), C (2), C again (3), D (4). You only count each letter name as you pass it. B (1), C (2), D (3). That gives you a 3rd.

The 4th Interval

A 4th is an interval that spans 4 note names. For example, E–A is a 4th because it spans 4 notes in the scale: E (1), F (2), G (3), and A (4).

You count all intervals in the same manner...5ths, 6ths, 7ths, and 8ths. Start counting from 1 and just count the letter names until you have reached the higher note.

For example, C–G is a 5th because it spans five letter names: C (1), D (2), E (3), F (4), G (5). The same counting method works no matter how large the interval gets.

The 1st Interval

The "1st interval" is also called the unison interval because there is only one note name involved. In other words, it’s the same note. On the guitar, you can utilize the unison interval to help you tune your guitar. For example, when you play the 5th fret of the E string, it produces the note A. This note A should be "in unison" with the open A string. In this case, A–A is a unison interval because you only span 1 note name.

That is the little trick behind standard tuning. The 5th fret of the low E string is A, and the open A string is also A. Same letter, same pitch. A–A is a unison.

The 8th Interval

The "8th interval" is also called the octave. The word “octave” comes from “oct”, meaning eight (think of an octopus and its eight legs). This is the same thing we’ve been calling the “lighter shade” of a note all along. So C–C′ (pronounced C to C prime) is an octave interval where C′ is the lighter shade of C (the note an octave higher).

Count the letters and you can see why it is called an 8th: C (1), D (2), E (3), F (4), G (5), A (6), B (7), and C′ (8). The first and last notes share the same name, but the last one is the lighter shade.

Scale Degrees vs. Intervals

You may have noticed something. Back when we learned scale degrees, C was 1, D was 2, E was 3, and so on. Now that we’re counting intervals starting from C, we get the same numbers: C–D is a 2nd, C–E is a 3rd, C–F is a 4th. So a lot of beginning guitarists (I see this all over the guitar forums on Reddit) assume that scale degrees and intervals are the same thing. They are not.

Here’s the difference. A scale degree is a note’s fixed address inside a key. In the key of C, E is always the 3rd degree, period. It doesn’t matter what other note you happen to play it with. An interval, on the other hand, is relative. It’s the distance between any two notes, and you can measure it starting from any note you want, not just the tonic.

The numbers only line up when you happen to count from the tonic. Step off the tonic and they come apart. Take D and F. As scale degrees in the key of C, those are 2 and 4. But as an interval, D–F is a 3rd (D, E, F). The interval doesn’t care that D is the 2nd degree. It just measures the distance from D up to F. In fact, a 3rd can be built off of any note: C–E, D–F, E–G, and so on. They are all 3rds.

Here is the clean way to separate them. Scale degrees are anchored to the key: in C Major, C is 1, D is 2, E is 3, and that stays true all day. Intervals are measured from wherever you start. C–E, D–F, and E–G are all 3rds, even though they start on three different scale degrees.

So here is the takeaway: scale degrees are fixed (they are always counted from the tonic), while intervals are movable (they can be measured between any two notes). They happen to match only when you start counting from the tonic, and that coincidence is exactly what trips people up.