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 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:

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

Other ways of identifying thirds

• Associating 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 third from the I to III degrees in major scales and the third of a major chord are major, we know then that D-F# is also a major third.
• Memorizing all major and minor thirds. Start with major thirds and continue with 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 (not recommended).

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