A Picardy third (also known as Tierce de Picardie) is a harmonic device used in European classical music.

It refers to the use of a major chord at the end of a musical section in a minor key. This is achieved by raising the third of the expected minor triad to create a major triad. The aural effect of this is unexpected on the ear, and can add a "bittersweet" feeling to the music.

For example, instead of a cadence ending on an A minor chord containing the notes A, C and E, a tierce de Picardie ending would consist of an A major chord containing the notes A, C# and E. Note that the minor third between the A and C of the A minor chord has become a major third in the tierce de Picardie chord.

The Picardy third does not necessarily have to be found at the end of a section — it can be found at any perfect cadence although it may also occur as the final chord in a plagal cadence. There is no such device as the "Inverted" Picardy third, where an expected major chord is replaced by its minor equivalent. (The final sections of some works, for example the first symphony of Samuel Barber certainly end in a way that strongly suggests and then belies an ending in major, but this is not the same thing, even using a Picardy third before the end of the work as part of the dramatic process, but this is not what an inverted Picardy third would be.) (Courtesy of Wikipedia)

Picardy third is a French word meaning sharp or pointed. It refers to the practice from the 16th century Baroque era of ending a composition with a major chord, when the rest of the composition is in a minor key, thus giving the composition a sense of finality. For example, a minor key song ends with the major version of the tonic (an A minor piece ends with an A major chord).

An example is the ending to The Beatles' And I Love Her, as shown below, that ends on the D major chord while the song is in the key of D minor.