A chord can be defined as a group of 3 or more different notes played together. Every chord is based upon a specific formula which relates back to the major scale after which it is named. The formula for a major chord is 1 3 5, hence the C major chord consists of the first, third and fifth notes of the C major scale, i.e., C-E-G.
A chord must contain at least 3 notes, and any of these three may be repeated. In the F chord illustrated there are 2 F notes, 1 A note and 1 C note.
Now consider the D major chord, which is constructed from the D major scale:
The same formula applies (1 3 5), so the notes of a D major chord will be D F# A.
Applying the 1 3 5 major chord formula to the scales outlined on page 130 gives the following chord notes:
* The root note is the note after which the chord is named (e.g., C is the root note of the C major chord).