Most of the time, songs are based on the common diatonic scales, which are major, minor and seventh chords. These chords can all be formed by notes within one octave of the scale they are based on.
The notes used for all these chords are 1, 3, 5, 7. Of course, in a minor chord, the triad will have a flattened 3rd note, and there are various combinations of flattened and raised notes for 7th chords, such as Minor, Major and Dominant sevenths.
Extended chords make use of the notes that are normally left out of Seventh chords, namely 2, 4 and 6.
When playing these extra notes, extended chords are formed. They are classed as “extended”, because the second, fourth and sixth notes are normally played in the next octave, thereby creating 9th, 11th and 13th chords. The reason for this is to avoid playing notes that are too close together in the chord, especially if there is only a half-step between two notes.
Extended chords are used to create tension in a song, or to resolve or lead to the next chord progression. They are more complex in nature than the standard major or minor chords, so they should be played with a clean guitar sound. If all the notes are included in the voicing, then they would sound muddy, or plain terrible when using even mild distortion.
There is a difference between chords that have extended notes added to them and full extended chords. If the chord is shown as a Major 9th of a particular key, then it is implied that the full voicing is required, so the chord is structured as a Major Seventh, with a ninth note added, to extend the chord.
If a ninth note is simply added, then the normal triad is played, be it minor or major, with the extra ninth note added. The seventh note is not included in the chord. As an example, Cadd9 means C Major, with an added ninth note, whereas CM9 is a C Major Seventh with a ninth note.
It is pretty difficult to play all the notes of an extended chord on the guitar at once. This is because of the limited range on the fretboard, and the awkward voicings, depending on the scale, and where its root is. This is why the Dominant, or fifth note is almost always left out. It is not important to how the chord sounds, because the third, seventh and extended 9th, 11th or 13th give the chord its “flavor”.
There are many variations on the basic extended chord. Once you have learned to form major extension chords, you can create minor extended chords by dropping the 3rd note by one step or fret. Raising or lowering the eleventh, ninth or thirteenth note can create a much more dissonant, interesting sound.
Play around with the extended notes, once you’ve formed your chord. By altering the last note during repeated strumming of the chord, you can make a guitar riff or progression much more interesting to listen to, and create complex-sounding harmonies if you are accompanying another guitarist.