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 lower and higher notes and a middle note inside the third. For example, the third C-E has two seconds: C-D and D-E. Using the following table we can find out the quality of the third:

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

Following this method we find that the third C-E is a major third because 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:


Other ways of identifying thirds

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


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José Rodríguez Alvira.