Fig 1. The C major scale
The triad formed using the C note as the root would consist of C(root), E(third) and G(fifth), and these comprise a C major chord (the designation major denotes the fact that the E is four semitones (a major third) higher than the root note.
Fig 2. C, E and G - The C major triad
Using the same scale (and thus, implicitly, the key of C major) a chord may be constructed using the D as the root note. This would be D(root), F(third), A(fifth), which comprise the chord of D minor, since the third, F, is a minor third (three semitones) higher than the root.
Other chords may be constructed in a similar way, taking each note in turn as the root i.e.
The dominant seventh
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