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The Professor
Musical Interval Lingo

One of the most common questions I get when it comes to talking about notes and intervals is, “How come a 2nd and 9th are the same thing, and when do I say 9th and when do I say 2nd?”

To help answer these questions, here is a short primer on intervals when it comes to scales vs. chords, which should help tackle any issues when it comes to using different names for the same interval in any given key.

The short answer is that when you are talking about scales you use the numbers 1 to 8 for each note in the scale, so a C major scale would be 1-2-3-4-5-6-7-8.

Then, you use the numbers up to 13 when talking about chords. So, the notes of C major expressed as a chord in numbers are 1-3-5-7-9-11-13.

Since scales are built in steps and use step-wise motion, we use ordered numbers, from 1 to 8, to talk about those notes.

But, as chords are built in thirds, in that you stack every 2nd note on top of each other to produce the chord, we use stacked numbers from 1 to 13 to talk about the notes of a chord.

Here is an example of a C major scale written as both a scale and as a chord, with the corresponding numbers below each note.

Click to view attachment

As you can see, the numbers 1-3-5-7 line up to the same notes in either case. But, 2 and 9, 4 and 11 and 6 and 13 are the same notes but different numbers.

This means that if you are talking about the note D in the C major scale you would call it a 2nd, but if you were talking about the note D in a C chord you would call it a 9th, the smaller number is used for the scale the larger number for the chord.

So there is the short answer to the question, if you are talking scales use the numbers 1-8, if you are talking chords use the numbers 1-13.

If you want to dig into these concepts further, here are some articles for further reading that you can explore in your own study.

Theory Quick Start Guide

Musical Intervals For Guitarists

Mastering Major Scales for Guitar

Mastering Guitar Chord Theory
when you are using the number 1-13 to talk about a chord, why is it only odds? when talking about a scale you use all of the numbers. is it to keep it non chromatic because that would make the chord sound bad?
Kristofer Dahl
Since chords are built by stacking thirds (=every other note starting from the first) - you won't come accross 2, 4, 6 etc

However those notes are still contained within chords, because:



So this is mostly a theoretical question, it's not that relevant when actually playing and constructing chords. And it does not relate to what you said about non chromatic.

So the exact same notes are used when constructing the major scale, and when constructing chords from the major scale.

Let me know if this makes sense?
+1 to what Kris just mentioned.

*If you like you can think of it this way too ...
For chords: 1 3 5 (7) make up the fundamental chord (maj, min, dom) and 2 4 6 can be extensions (9th, 11th and 13th).
*We generally name/number the extensions above the octave (8) because most of the time (at least theoretically) they are played above the basic chord.

Chords within the scale: 1 3 5 is the I major, 2 4 6 is the ii minor, 3 5 7 is the iii minor, 4 6 8 is the IV major, 5 7 9 (or 2) is the V major, etc.
Odd > even > odd > even ...
I am discombobulated blink.gif

Kristofer Dahl
Hehe which part are you having problems with?

If everything feels discombobulating to you, this is not the right place to start.

You should first understand how chords and scales are constructed, before this topic will be relevant for you. Check out the links in The Professors opening posts to learn more about the basics.
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