Harmonising the Major Scale
In this week's lesson we're going to look at how to harmonise the Major Scale using Major and Minor chords as well as 7th chords. What does harmonise the Major Scale mean though? It simply means playing the notes of the Major Scale but using the correct matching chords to bring out the sound of the scale. First we will harmonise the Major Scale using Major and Minor chords before moving on to harmonise the Major Scale using 7th chords.
Harmonising using Major and Minor chords
First we're going to look at how to harmonise the Major Scale using mainly only Major and Minor chords. There is a rule though and luckily it follows the same rule for every key. To harmonise the Major Scale you use these chords:
Chord I: Major
Chord II: Minor
Chord III: Minor
Chord IV: Major
Chord V: Major
Chord VI: MInor
Chord VII: Diminished
And here's how it would look harmonising each note of the C Major Scale:

And it doesn't matter what key you're in. In every key it'll follow the same rule of Chord I being Major, Chord II being Minor, Chord III being Minor, Chord IV being Major, Chord V being Major, Chord VI being Minor and Chord VII being diminished. So being in the key of G Major the chords would be G Major, A Minor, B Minor, C Major, D Major, E Minor and F# Diminished. It's the same for every key. Now let's move on to harmonising the Major Scale with only 7th chords...
Harmonising the Major Scale using only 7th chords
Harmonising the Major Scale using 7th chords we'll be using Major 7, Minor 7 and Dominant 7 chords instead of straight Major and Minor chords. To harmonise the Major Scale we use these chords:
Chord I: Major 7
Chord II: Minor 7
Chord III: Minor 7
Chord IV: Major 7
Chord V: Dominant 7
Chord VI: Minor 7
Chord VII: Minor 7 b5
And here's how it would look like if we were to harmonise the C Major Scale using only those 7th chords:

And it would be the same in every Major key. In each Major key Chord I will be Major 7, Chord II will be Minor 7, Chord III will be Minor 7, Chord IV will be Major 7, Chord V will be Dominant 7, Chord VI will be Minor 7 and Chord VII will be Minor 7 b5. So let's say we're in the key of A Major. If we want to harmonise the notes of the A Major Scale with only 7th chords Chord I will be A Major 7, Chord II will be B minor 7, Chord III will be C# Minor 7, Chord IV will be D Major 7, Chord V will be E Dominant 7, Chord VI will be F# Minor 7 and Chord VII will be G# Minor 7 b5.
Summary
So to summarise make sure you learn this theory and which chords can be used to harmonise which notes in any given Major Scale. The good thing about the theory here is that it makes writing your own chord progressions and pieces of music much easier as it's already given to you which chords you can use in any given key. Rather than come up with a chord progression by luck all you've got to do is memorise the theory and you can pick and choose which chords you want and they will always sound good. A good exercise is to pick any key you want and start writing chord progressions using the Chord theory I've given you here. Have fun!