Triads are three note chords consisting of a Root, Third, and Fifth. The numbers are derived from the position on the scale. There are four common types of triads: Major, minor, diminished, and Augmented.
The first, third, and fifth degree of the Major scale a group of pitches arranged from high to low covering one octave form the Major triad a three note chord consisting of a 3rd and 5th above a root: Major, minor, diminished, Augmented, see Chord Construction or Major chord a group of three or more tones forming a harmony. The Major chord has a M3rd and a P5th above the root.
Lower the third of the Major triad to make the minor.
c diminished, c dim
Lower the third and the fifth of the Major triad to make the diminished.
C Augmented, C Aug
Raise the fifth of the Major triad to make the Augmented.
Seventh chords consist of a Root, Third, Fifth, and Seventh.
C Dominant 7 a Major chord with a m7th built on the fifth degree of the Major or minor scale, V7, C7
C Major 7, CMaj7
C minor 7, Cm7
C half diminished or C minor 7 flat 5, Cm7b5
C diminished 7, C dim7
C7 flat 5, C7b5
C minor Major 7, CmMaj7
Dominant Ninth Chords
Dominant ninth chords can be built by stacking another third on top of the dominant seventh chord. It is common to omit the fifth of the ninth chord.
see also: Keyboard Chords, Staff Name Chords, String Chords, Chord Inversions