What is a Tierce de Picardie?

A Tierce de Picarie ('Picardy Third') is a surprise major chord at the end of a piece. Whilst the piece is predominantly in a minor mode, the final chord (which will always be the tonic in music of this period) has an altered (sharpened) third, making it sound major. It was common in the Baroque period, and adds a sense of 'opening out' and optimism to the piece's final moments.