The painting consists of 12 panels, four in the centre and four on each of the folding wings. The lower tier depicts the spirituality of the world, and God’s chosen people; the upper tier shows the heavenly realm with Adam and Eve on either end.

The focus of this panel is the Lamb of God, spurting blood on an altar. Four sets of figures approach: virgin martyrs; figures from the New Testament and the Church; patriarchs and prophets of the Old Testament; and confessors of the Faith.

The central figure of the upper tier is God, depicted in a brilliant red robe and a bejewelled mitre, carrying a sceptre and with a crown at his feet. The benign calm and poise of the face radiate throughout the polyptych.

The numerous flowers make a philosophical point: everything in nature is an expression of God’s work. The painter’s job was to record it faithfully.

The figure of Mary tells us much about the concept of feminine beauty in medieval times. Fine-featured, absorbed in her reading, she is decked with jewels.

A heavenly choir sings on one side of the upper tier, while on the other, an orchestra of angels plays. The figures are tightly crowded, but the perspective is good.

Jan van Eyck’s contemporaries were startled by the realism of his Adam and Eve. Even today, their nudity among the luxuriously clothed figures is striking. Beautifully lit from the right, they show the painter’s profound understanding of the human form.

To the rear of the central panel rise the towers and spires of the heavenly city, Jerusalem.

In the 19th century, a verse inscription by the two brothers, thought to be original, was uncovered on the frame.

The wings of the painting can be closed. The external panels are tonally quite flat, intensifying the moment they are opened to reveal the sumptuous interior.