Basic Chord Formation

Chords are formed by playing 3 or more notes at the same time. All chords are made up of notes from a scale.
Here is the C major scale

C D E F G A B C
1 2 3 4 5 6 7 8

In this example the notes that are played together are the bold ones C E G or if you use numbers, 1 3 5. This is a basic 3 note chord (triad).

As you can see numbers are used to represent the notes of the scale, 1 3 5 being the 1st, 3rd and 5th degree of the scale.

You can make inversions of the chord, for example instead of:

1 3 5 = C E G you could try 3 5 1 = E G C or 5 1 3 = G C E

By making inversions you are adding colour, but it is essentially still a C major chord because it is not the notes that have changed it is the order, the process of making inversions can be applied to all chords.

Q: What constitutes a chord?
A: 3 or more notes played together.

Here is the C Natural Minor Scale

C D Eb F G Ab Bb C
1 2 b3 4 5 b6 b7 8

By playing C Eb G or 1 b3 5 at the same time, a basic minor chord is formed:

In the C minor chord the 3rd note has been flattened by a semitone (b3). The 3rd is the note in a chord that indicates whether it is major or minor.

Here is the formation of a suspended 4th chord

C D E F G A B C
1 2 3 4 5 6 7 8

With the suspended chord there is no major 3rd present so the suspended chord could be major or minor, depening on what other melodic instruments are playing whilst a suspended chord is being played.

Three note chords (triads) played successively have their uses, but you can embellish the chords by adding other notes derived from the scale, an option is to add the 7th. In the next example we are going to do just that with the construction of a Cmaj7 chord.

C Major Scale

C D E F G A B C
1 2 3 4 5 6 7 8

Here is another example of a basic chord with an added 7th. The name of this chord is Cmin7

C Natural Minor Scale

C D Eb F G A Bb C
1 2 b3 4 5 6 b7 8

To create more texture lets add the 9th to form maj9 and min9 chords, remember these added 7s and 9s are notes derived from the respective scales, in this case the major scale and the natural minor scale.

C Major Scale

C D E F G A B C D
1 2 3 4 5 6 7 8 9

Observe that the 9th note is the same as the 2nd except an octave higher.

C Natural Minor Scale

C D Eb F G A Bb C D
1 2 3 4 5 6 7 8 9

Further Embellishment

Further embellishment is possible with b9, #9, 11, #11, etc... However, we are limited by how many notes we can play on the guitar simultaneously. Take a chord like C13, the theory of the chord is 1, 3, 5, b7, 9, 11, 13. To play all those notes at once on the guitar is a physical impossibility, so usually the 1, 3, b7, 13 are played; why those notes are chosen out of the entire 7 notes that theoretically make up a 13 chord is due to certain chord tones that need to be present to give the 13 chord its sound. In the case of the major and minor triads that we looked at earlier on, the 1st and 3rd notes have to be present so we can tell whether we are dealing with a major or minor tonality, then the b7 signifies that the chord is a type of dominant and the 13 is the added upper partial that gives the chord a characteristic indicative of its construction.

Important note!

The major scale is the scale by which others are related, so when we talk about altered notes b3, b6, b7, #9 and so on, those notes are altered in relation to the major scale. If it is b6 it means that the 6th note of the major scale is flattened by a semitone, if it is b7 the 7th note of the major scale is flattened by a semitone and when you encounter #11 it means the 11th note of the major scale (which is the 4th note of the scale an octave higher) is sharpened by a semitone.