Harmonising the Major Scale

We can make chords out of every note of the major scale by adding triads on to each note in the scale, using only the notes in that scale. For example, if we take the A major scale:

We know that the root note (A) becomes an A major chord when we add the 3^rd (C^#) and 5^th (E) notes to it and play them simultaneously.

• If we take the 2^nd note (B), and do the same thing, but this time using the B as a starting point, we will come up with the 2^nd (B), 4^th (D) and 6^th (F^#) notes as our triad (which is the 1^st, 3^rd and 5^th notes relative to B). If we look at this triad closely, we find that it is a minor chord, since it contains a minor third (B to D = 3 semitones). So we get B minor.
  
  
• The 3^rd chord (C^#) will contain the 3^rd (C^#), 5^th (E) and 7^th (G^#) notes of the scale. This triad also contains a minor third (C^# to E) and so we get C^# minor.
  
  
• The 4^th chord (D) will contain the 4^th (D), 6^th (F^#) and 1st (A) notes of the scale. This is a major triad, so we get D major.
  
  
• The 5^th chord (E) will be composed of the 5^th (E), 7^th (G^#), and 2^nd (B) notes. This is a major triad, so we end up with E major. This is also the dominant chord in the key of A, so we can substitute E7.
  
  
• The 6^th chord (F^#) contains the 6^th (F^#), 1^st (A), and 3^rd (C^#) notes. This is a minor triad, so we get F^# minor. This is also the relative minor of A major.
  
  
• The 7^th chord (G^#) contains the 7^th (G^#), 2^nd (B) and 4^th (D) notes. This is a diminished triad, since both the 3^rd and the 5^th are flat, so we get G^#minor^b5.

Listen:

These chords are the same across all major keys. So, in any major key, we will get the following chords:

The chord built on the 1^st note of the scale is MAJOR
The chord built on the 2^nd note of the scale is MINOR
The chord built on the 3^rd note of the scale is MINOR
The chord built on the 4^th note of the scale is MAJOR
The chord built on the 5^th note of the scale is MAJOR (dominant)
The chord built on the 6^th note of the scale is MINOR (relative minor)
The chord built on the 7^th note of the scale is DIMINISHED (minor^b5)

This is useful to know because it gives you a set of chords that are diatonic, or ‘in key’ since they only contain notes that are in the major scale of the key you are playing in. So you know that any of those chords will sound good together in that key. In fact, most popular songs which stay in a single key will almost exclusively use these chords from the key they are playing in.