Major Third

A C major chord contains C, E and G. C is the root, so it gives the chord its name. G is the fifth. It adds stability and weight, but it does not tell you whether the chord is major or minor.

E does. The distance from C to E is a major third. Move E down one semitone to E flat and you get C minor. C and G stay exactly where they are.

That one-note comparison is the clearest way to hear what musicians mean by the color of a third. You may hear C major as brighter and C minor as darker. Those words describe the contrast; the concrete change is E moving to E flat.

Play C and G first, without a third. The two notes form an open fifth. Now add E, then replace it with E flat. The root and fifth provide the frame; the third gives that frame its clearest major or minor character.

The number and the quality describe two different things.

Count the letter names from C to E, including both ends: C, D, E. That makes it a third. Then count the semitones from C to E. There are four, which makes it major.

C to E flat still uses three letter names, so it is also a third. It spans three semitones instead of four, which makes it a minor third. If you need the full counting method, start with What Is an Interval?.

Play C to E, then D to F. Both pairs span three letter names. Their widths are different.

C to E covers four semitones, so it is a major third. D to F covers three, so it is a minor third. The letter names determine the number; the semitone count determines the quality.

Spelling still matters when two intervals use the same piano keys. C to E flat is a minor third because it spans the letters C, D and E. C to D sharp sounds the same on a modern piano, but it spans C and D, so its written name is an augmented second. The notation tells you how the notes function in the music.

A major chord is named for the major third above its root. That interval stays in place when other notes are added or changed:

Minor chords use a minor third above the root. Sus chords replace the third with the second or fourth, so they do not state major or minor in the same direct way.

A minor chord can still contain a major third between two of its other notes. C minor is C, E flat and G; E flat to G is a major third. When you identify a chord as major or minor, measure from the root.

Measure from the root of a scale to its third degree. Major, lydian and mixolydian all place a major third there.

That tells you they belong to the major side of the family, but it does not make them interchangeable. Lydian raises the fourth degree; mixolydian lowers the seventh. The third answers the first question, major-type or minor-type, and the other scale degrees tell you which scale it is.

Natural minor, dorian and phrygian place a minor third above the root. When you are identifying a scale by ear, finding the third narrows the possibilities before you listen for the notes that distinguish the modes.

A pure major third has a frequency ratio of 5:4. For every four vibrations of the lower note, the upper note completes five. Their vibration patterns line up regularly, which reduces beating between their harmonics. This version measures about 386 cents. A cent is one hundredth of an equal-tempered semitone.

A modern piano uses equal temperament, which divides the octave into twelve equal semitones. Four of those semitones make a major third of 400 cents, about fourteen cents wider than the pure 5:4 version. Hold a piano third and you may hear a gentle shimmer instead of a completely still blend.

The compromise lets the piano play in every key without being retuned. Singers and string players are not tied to fixed keys, so an ensemble can lower the upper note slightly when it wants a purer blend. The full explanation is in Tuning and Temperament and The Harmonic Series.