A triptych is a style of painting in three panels of wood which makes painting much more vibrant and are hinged together side by side, used as an altarpiece. The links in the wooden panels make a painting to open or shut close. Some of the famous panel painting is the Merode Altarpiece, the Portinari Altarpiece, the Raising of the Cross so on.
The particular piece of music is the third piece in the composers 20th collection of works. That can included all kinds of things like symphony's operas among others.
