Two versions of Leonardo's celebrated "The Virgin of the Rocks" are to be displayed together for the very first time at the National Gallery's forthcoming blockbuster exhibition "Leonardo da Vinci: Painter of the Court of Milan," running from November 9 to February 5 of 2012. Both paintings, one owned by the National Gallery and the other by the Louvre, represent a Madonna and child flanked by an infant saint — probably John the Baptist — and an angel. They were painted within 20 years of each other, but details of how, and why, the Renaissance master created the two nearly identical works remain obscure to this date.

In 1483 the Confraternity of the Immaculate Conception in Milan commissioned Leonardo to paint a virgin and child destined to be the main panel of an altarpiece. Ten years into the project, the artist entered a bitter dispute with his commissioners over money. "I think probably what happened is that Leonardo realized that he had painted something really amazing," says Luke Syson, the Gallery's curator of pre-1500 Italian paintings and head of research. "It's the first really large-scale picture that he actually completed." Perhaps judging that it was worth more than what the Confraternity was ready to cough up, Leonardo sold the painting to someone else around 1491-2.

This picture, now part of the Louvre collection, reemerged in France in the early 17th century. Meanwhile, Leonardo started working on a new panel for the Confraternity, which is the version now held by the National Gallery in London. He delivered it, unfinished, in 1499, when he escaped Milan after the fall of the Duke Sforza, and then came back to Italy in the first years of the 16th century to add what Syson calls "perfunctory finishing touches."

To shed light on the mysteries surrounding two of Leonardo's best-known paintings, ARTINFO UK asked Syson to analyze the differences between the versions.

FOREIGN HALOS

"The halos in the National Gallery painting were almost certainly added by somebody other than Leonardo, perhaps a more literal-minded member of the Confraternity, probably when the picture was moved in the late 16th Century. For Leonardo, the mise-en-scène — the mysterious setting — and the extraordinary beauty of the figures were enough to make the representation holy. This detail gives you a sense of the tensions between the piece as a work of art and as a functional part of an altarpiece."

JOHN THE BAPTIST, OR ANOTHER BABY?

"It's extremely difficult to tell who is who in this painting, especially of the little boy on the left-hand side in the Louvre picture. John the Baptist has absolutely none of the normal attributes that he would have. He doesn't have a halo, he doesn't have his little scroll, he doesn't wear a camel skin tunic, and he isn't carrying a cross. He's also higher up in the picture than Christ. We are not sure as to what on earth he's doing there, or if it truly is John the Baptist. There's no mention of John the Baptist in the contract from 1493, so why he is there, and why he has been retained in the London picture is really a mystery."

BETTER AND WORSE ANGELS

"Angels are theologically sexless, and Leonardo was very keen on an ideal beauty which wasn't dependent on gender. So the depiction of an angel for him is a rather satisfactory ideal of pure sexless beauty. The angels in both pictures are extraordinary, and, again, very mysterious. The angel in the Louvre painting is wearing a more brightly coloured costume. It is looking out at the viewer and is pointing towards the little boy who might be John the Baptist, signalling his importance."

"People have suggested that it might be an archangel, but it seems to me that it's too small. The angel's hand pointing in the Louvre picture is a distracting element. It interrupts the gesture between the virgin and her son. So it's not very surprising, I think, that Leonardo chose to leave that the hand out in the second version, the National Gallery painting. The mystery is why he included such a literal sign in the first place. One suspects it might be because he was instructed do so. It doesn't feel like his own decision."

TAKING UP PICTORIAL SPACE

"Although the same thing more or less is happening in the two pictures, you start really understanding Leonardo's different approaches when you start thinking about the landscape. In the National Gallery painting, the landscape is one of the less finished aspects. There are bits that are quite unresolved, and this could be explained by the fact that the artist might have been looking to produce the second version more cheaply. In the Louvre picture, Leonardo put the protagonists on a little stage, a space that you can identify. In the National Gallery picture, everybody looks like they might tumble out into our space. It is much more about the idea of the sculptural relief, making something present in our own world."

DIVINE LANDSCAPING

"The plants and the rocks are also very different. Whereas in the Louvre painting it's all about close observations of nature — we have a lush, damp grotto with irises, all beautifully observed — in the National Gallery painting Leonardo is making them up. He's taking different kinds of plants and putting them together. In the Louvre painting, Leonardo pictures the world as people experience it, whereas in the National Gallery painting, Leonardo pictures an ideal world. This is an indication of a shift in Leonardo's own thinking — the painter as mirror of nature becoming the painter whose creativity is akin to the creativity of God."