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Introduction

In our previous harmony lessons, we've built chords by using notes that are found within a single scale. We've limited our choice of notes to just the first octave of the scale. For example, a C Major chord is constructed from the root (first), third and fifth notes of a C Major scale:

If we add the 6th note of the scale to the chord, we now have a C6 chord:

If we add the 7th note of the scale, we have a C Major 7th chord:

The 8th note of the Major scale is the same note as the root note, just one octave higher. But if we add to the chord a note that is above the 8th note, such as the 9th note, we have created an extension to the chord. This chord now becomes a 9th chord.

As you can see in the chart below, the 9th note in the key of C Major is the note D. The 9th is always the same note as the 2nd note in the same key, except that it is one octave higher: