A chord is made up of 3 or more notes from the major scale, for e.g., the major chord is made up of the 1st, 3rd and 5th notes of the major scale. To form the C major chord the 1st, 3rd and 5th notes of the C major scale are required.
As shown in the previous C scale diagram the notes needed are C, E and G. To form the G major chord the 1st, 3rd and 5th notes of the C major scale are required. As shown in the previous G scale diagram the notes needed are G, B and D.