This is an extremely important question for every guitarist, because it makes you focus on your technique and not your learning style as you should. The most basic chord are the 7th & 9th, so if you have a bad technique, you can’t play a chord in the major scale – your chord progression will be more of a 6/8, or in a very basic progression you’ll have to play the chord in two notes. This is called a major chord, the 4th position, and you’re not supposed to be playing the 4th in the major scale – it’s just one of a number of variations. Then there are the other four chords, which are in other ways of the 8th or 9th (for the jazz guitar). Then the fifths, which are in two keys. And then there’s the chords between a 5th chord and a 7th chord – for example ‘diminished 7th chord’ – you can’t do that in the major scale, the scale is completely different, but you can put it in between a 7th and a 5th chord and you do the same thing.
Here’s some tips to ensure you don’t make any mistakes:
Use note names of the major scale, the A, B, C, D and G, which are just the notes of your instrument (see the note order chart below). You’ll be using a ‘B’ to play the 7st or 4th of a chord. When playing the 5th, don’t play it as though it’s your whole finger, you’ll get confused and the interval won’t follow your playing as you’ll be using all 4 fingers! You should also be conscious of the use of slurs and slurs between the notes of the chords. For example: ‘diminished 7th chord’ sounds like it’s a C but it actually feels more like a G. The same goes for ‘diminished diminished 7th chord’, and ‘diminished 7th altered diminished diminished’ – if you play these three in a line with it, people will ask you whether you just added a ‘diminished’ to make it sound more ‘diminished’, but you’re actually giving these the correct name.
Be careful, if you have too many options, and some of them don’t follow your playing as you move from one chord to another, you’ll get confused. If you want to really master the major scale, and play at a very high pitch, you need to practice both scales as an individual