Diptych triptych form, with two outer panels that could be closed across the main central representation. They are one type of the multi-panel forms of painting known as "polyptychs". The diptych was a common format in Early Netherlandish painting and depicted subjects ranging from secular portraiture to religious personages and stories. Often a portrait and a Madonna and Child had a leaf each. It was especially popular in the 15th and 16th centuries. Painters such as Jan van Eyck, Rogier van der Weyden, Hans Memling and Hugo van der Goes used the form. Some modern artists have used the term in
Saint Peter Martyr Altarpiece testimony) also mention a second altarpiece by the artist in the same church, also dating to 1476, now known as the "Saint Peter Martyr Altarpiece"'. It is therefore theorised that the altarpieces were both dismantled during rebuilding of the church in 1776, then sold to the antiquarian Grossi who sold all the panels separately and grouped some of them into the altarpiece that Cardinal Zelada bought from him. Federico Zeri and Rodolfo Pallucchini then joined the altarpieces together, initially referring to the first (the high altarpiece), the "Pietà" in the Metropolitan Museum, and then to the "Madonna" in the Museum