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An example of mode mixture, the harmonic device of a picardy third (Tierce de Picardie) is the use of raised third in the final tonic chord of a musical composition or section in minor, the use of which originated in the 16th century and the Baroque era and was common up to the eighteenth century. It makes the final chord a major triad or chord rather than the expected minor triad of the natural, melodic, and harmonic minor scales. The aural effect is unexpected on the ear, and can add a "bittersweet" feeling.
The "Picardy third" translation describes the change of the third in the chord - for example, instead of ending on an A minor chord with A, C, E, with a tierce de Picardie the chord will be A, C#, E, with the minor third between the A and C becoming a major third.
The interval strength, or lowest possible location in the harmonic series, and thus consonance and "stability", of minor triads is less than that of major, which interprets major as more "stable", a major triad being found in the 4th, 5th, and 6th harmonics of a pitch, while the minor being the 5th, 6th, and 7th. This may explain the picardy third, since it would be more strong and stable, and thus conclusive. Also supporting this, the picardy third is still considered part of an authentic cadence, the strongest cadence. (A picardy third may occur as the final chord in a plagal cadence, also.)
Here is an example from Johann Sebastian Bach's cantata Ich habe genug.
Before the singer begins the aria in this section, we have the continuo playing this cadence. What makes this cadence a tierce de Picardie is noted by the red natural sign: instead of this red sign being a flat, which would make the chord minor; we have a natural, making this chord major.
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