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Theory

Parallel thirds

Parallel thirds are two voices that move up or down together a third apart. They sound soft and sweet and, unlike parallel fifths or octaves, count as allowed. You hear them in duets, in folk song and in pop music.

Meaning
two voices a third apart, same direction
Status
allowed, unlike fifths and octaves
Within the key
major and minor thirds alternate
Typical of
duets, folk songs, choral writing

Soft, but not fused

The third is an interval that does not fuse quite as strongly as the fifth. That is why two voices a third apart remain audible as two voices even when they run in parallel. It is the reason why harmony teaching avoids parallel fifths but expressly allows parallel thirds. They are the most common special case of parallel motion.

Thirds in C major, ascending, with a closing chord

4 4

A scale in thirds: C–E (major third), D–F (minor), E–G (minor), F–A (major), G–B (major). At the end the line goes back down with E–G. The distance in scale steps stays the same, but the size of the third changes, because the semitones of the scale fall in different places.

Major and minor thirds

Within a scale the size of the third changes from degree to degree. In C major there are three major thirds (on C, F and G) and four minor ones (on D, E, A and B). The key is the framework here, not the distance of a third alone: the voices keep the same distance in scale steps, even when the size changes. The link between the size of the third and major and minor is the subject of the article "Major and minor: the third as a switch".

A long chain of thirds becomes monotonous after a while. Textbooks therefore recommend breaking it up after a few steps, for example with contrary motion or with sixths.

Practice: Play the example with two instruments or with both hands. Check where you hear a major and where a minor third. In Interval Ear Training you practise exactly this difference.

Common mistakes

  • Forbidding parallel thirds: Only fifths and octaves are a problem in strict teaching.
  • Treating the size of the third as constant: It changes with the semitones of the scale.
  • Writing chains that are too long: They tire the ear.

Continue on Maikstrings

Interval Ear Training

Tell major and minor thirds apart by sound.

Major and minor: the third as a switch

Why the third decides between major and minor.

Identify Chords

Tell major and minor apart by sound.

Related terms

Parallel motionParallel fifthsThirdMajor thirdMinor thirdSimilar motionInterval

Editor: Maik Krohm · reviewed on 3 October 2026

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