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:

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

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

Diminished third, 2 half steps:

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

• Associate thirds with scales, triads, etc. For example, the third D-F# can be associated with the I and III degrees of the D major scale, or with the third of the D major triad. If we know that the thirds of the I through III degrees in major scales and the third of a major chord are major, then we know that D-F# is also a major third.
• Memorize all major and minor thirds. Start with major thirds and move on to minor thirds. You will learn them with practice.

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