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 G—B is a major third.
• Minor Third (m3)
  Equal to three half steps; G—B♭

*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.)

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 

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

• G Major Triad
  Root Note: G
  Major Third: B
  Perfect Fifth: D
• G Minor Triad
  Root Note: G
  Minor Third: B♭
  Perfect Fifth: D