Third

Two sizes

The name comes from the way of counting: from C to E lie the three degrees C, D, E. The third therefore skips exactly one note of the scale. Whether it is major or minor is decided by its width in semitones.

In just intonation the major third has the frequency ratio 5:4, the minor third 6:5. Together they make a perfect fifth (3:2). On the guitar the thirds correspond to three or four frets apart on the same string.

The third decides between major and minor

A C major triad consists of C, E and G, a C minor triad of C, E♭ and G. Two of three tones are identical, the third is shifted by a semitone. That is the smallest difference the system offers, and it decides whether a chord feels radiant or shaded.

How the third switches the character and how composers use this is explained in the article Major and minor: the third as a switch. The fifth on its own also sounds empty, because it lacks the third: C–G is neither cheerful nor sad. Why that is so is in the article Why the fifth sounds hollow.

Tuning thirds

In equal temperament the major third lies 14 cents higher than in just intonation. You hear this as a slight beating. Ensembles with flexible pitch, such as string quartets or vocal groups, therefore automatically play and sing thirds a little lower than the piano would. More in the article Equal, meantone, just: why your piano is slightly out of tune.

Common mistakes

• Confusing major and minor third: A minor third has three semitones, a major third four. In the major triad the major third is at the bottom.
• Equating the third with the third degree: The third is the distance to the third degree, not the degree itself. The note E is the third degree in C major, and the distance C–E is a major third.
• Treating the tuner as the yardstick: It measures against the equal-tempered grid, that is, against a compromise. A third played in just intonation deliberately departs from it.