The Importance of the Third Interval

A third is an interval equal to three scale degrees that is most commonly major or minor*:

  • Major Third (M3)
    Equal to four half steps; the distance between GB is a major third.
  • Minor Third (m3)
    Equal to three half steps; GB♭

*By itself, the word "third" often refers to the major third (it’s common to omit the word ‘major’ when speaking of intervals, chords, and keys.)

The Third as a Scale Degree

This refers to the third note or chord on a musical scale. When reading or playing a scale, the third note will be the first clue as to whether that scale is major or minor. Compare the following G scales:

  • G Major:  G - A - B - C - D - E - F♯
  • G Minor:  G - A - B♭ - C - D - E - F 

The Role of the Third in Chords

In many chord types, the third is an important building block. For example, major and minor chords are built similarly, except for their thirds: