Thirds can be major, minor, augmented and diminished. Below you can see that the number of half steps determines the quality of a third:

Major third, 2 whole steps or 4 half steps:

major third

Minor third, 1 1/2 whole steps or 3 half steps:

minor third

Augmented third, 2 1/2 whole steps or 5 half steps:

augmented third

Diminished third, 2 half steps:

diminished third

Identifying thirds

A third can be identified by analyzing the seconds between the upper and lower notes and a middle note within the third. For example, the third C-E has two seconds: C-D and D-E. Using the following table, we can determine the quality of the third:

If the seconds are: then the third is:
major - major major
major - minor minor
minor - minor diminished
major - augmented augmented

Using this method, we find that the third C-E is a major third, since both seconds (C-D, D-E) are major seconds.

If any note has accidentals, we can determine the quality of the interval without accidentals and then analyze the effect of the accidentals:

Example: Ab-Cb:


  1. Make all notes natural. A-B is a major second, B-C is a minor second, so A-C is a minor third.
  2. Add a flat to A. The interval is now a major third.
  3. Add a flat to C. The interval is now a minor third.

Other Ways of Identifying Thirds

Learning the number of steps for each type of third and counting the whole and half steps is not recommended.

See I > Intervals for related entries. To learn about go to Tutorials > Intervals.

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José Rodríguez Alvira.