A minor chord (or a "minor triad") is a chord composed of three notes, where the distance between the first and the second note is three semitones and the distance between the second note and the third note is four semitones.
The E minor chord, for example, is Em and consists of the notes E, G, and B. There are three semitones between E and G. There are four semitones between G and B.
The minor chord can also be described as composed of the minor third and the perfect fifth of some scale. In the minor scale in E, for example, the third note is G. G is a minor third, because the interval between the first note E and G is three semitones. The fifth note on the scale is B. It is a perfect fifth, as the interval between the first note E and B is seven semitones.
Examples of scales with minor chords
The following are examples of where the minor chord occurs in common heptatonic scales.
Examples of relationships between the minor chord and other three note chords
Shifting the second note of the minor chord one semitone up produces a major chord (i.e., switching from a minor third to a major third).
Shifting the third note of the minor chord one semitone down produces a diminished chord (i.e., switching from a perfect fifth to a diminished fifth).
Shifting the second note of the minor chord two semitones up or one semitone down produces a suspended chord (i.e., switching from a minor third to an augmented third or a diminished third).