There can't be many shows where the curator says that he can't wait for you to see the back of the works on display.
But that is what the National Gallery's new exhibition of Italian medieval and early Renaissance altarpieces does, the point being to demonstrate the way that these works were constructed.
It may seem nerdish but in fact it's enlightening and rather fun. Testing the wood, seeing the way its grain runs and where the dowels and joints were placed can help conservators tell where the bits and pieces of the great altarpieces, long since broken up for sale, were originally placed and how the whole structure once looked.
And there's an artistic point, too, in the way in which painting style changed as the construction moved from a piece made up of multi panels organised around vertical struts (polyptych) to one based around a single rectangular panel (pala), which allowed the artist to compose a picture across the whole surface.
Go to the front of the two altarpieces displayed with their backs on view and you can see the artistic development that took place. The polyptych, Giovanni dal Ponte's The Ascension of Saint John the Evangelist with Saints from the early 1420s, has Saint John in the middle flanked by two separate panels of figures on either side as in a triptych, a bottom sequence (the predella) of horizontal pictures of the saint's life below, as well as pictures of the crucifixion, annunciation and the Virgin above. From the life of the saint through the presence of Christ, or the Virgin Mary rising up, it's a gilded glorification of faith.
Look at the other work in the room, Francesco Botticini's San Gerolamo Altarpiece, painted some 70 years later, and you are in a different world of much freer painting. Although the central saint is again flanked by figures on either side, and is actually separated from them by a gilded frame, the colours and rhythm are as one, much closer in style to post-Renaissance religious paintings.
The National Gallery summer show, Devotion by Design: Italian altarpieces before 1500 is exactly what major museums should be doing. It is free. It uses the museum's own holdings, many of them from the basement reserve, to deepen understanding of the major works on display in the main galleries. It demonstrates what the curators and conservationists are up to at the "back of the shop" and there is a point to it other than just showing works of art.
In this case, the purpose is to provide the context in which the great altarpieces of the Italian Renaissance and pre-Renaissance were commissioned and placed. Upstairs, the masterpieces of Piero della Francesca, Mantegna and others are displayed as paintings, set against walls and framed like the artworks that succeeded them. Downstairs, the exhibition aims to show them as works with a purpose, set behind and above the altar to inspire priests as they raised the Host, which turned miraculously into the blood and body of Christ.
By accident or design, it could be regarded as a companion exhibition to the British Museum's current show of relics and reliquaries. The BM's works were objects to which pilgrims great and small would come to venerate and to absorb the holiness associated with a particular saint or relic. Altarpieces were also commissioned as an aid to spirituality but they were there to adorn and assist the regular liturgy of worship, not to become special objects of worship themselves.
How far the ordinary congregation saw more than a glimpse of the gilded frames and holy figures is another question. Divided from the chancel by the rood screen, they may have seen something of the gilded grandeur of these works at the high altar but not their detail. In this way, altarpieces were an elitist art, or rather art for the select, the clergy and dignatories allowed into the chancel to witness the miracle of transubstantiation or the side chapel to conduct a ceremony, and the monks and friars attending services in their religious houses.
It is no accident that most of these works come not from churches but from monasteries and abbeys, where the art was protected while the altarpieces from churches were disassembled, cut down and replaced as Protestantism forced the Roman Catholic Church to involve the laity more closely in services. It helps to explain, too, why the works included particular saints in received hierarchies with special emblems and attributes. Part of it, at a more mercenary level, was to keep the donors happy. There's a wonderful work in this show by Carlo Crivelli of La Madonna della Rondine, from the Ottoni Chapel in the church of San Francisco in Matelica in Marche. On the left is the figure of St Jerome with St Catherine below, as representatives of the Conventual Franciscans, joint patrons of the altarpiece. On the right is the figure of St Sebastian dressed as a soldier and below, St George representing the military attributes of the Ottoni family, the lay sponsors. Almost everything in the work had some resonance.
The effect of putting the pieces on show at the height and with the lighting in which they would have been seen is well illustrated by the staging, in the main room, of Luca Signorelli's The Circumcision from around 1490, several steps up, as it would have been at the high altar, and reaching towards the ceiling. It's not a great painting but the effect of looking up at it in full majesty is undoubtedly powerful. The symbolism is again important. The unusual choice of scene depicts the first blood spilt by Jesus and, by association the blood he was finally to spill on the cross as celebrated in "The Eucharist".
One of the pleasures of this exhibition is the number of pieces from the basement. Wisely, the curators have decided not base the show around any of the masterpieces from the upper galleries. Some two-thirds of the pieces on display are from the reserve collection, including Nicocolò di Pietro Gerini's splendidly restored Baptism Altarpiece from 1387, which served as a model for Piero della Francesco's The Baptism of Christ, two floors above.
And then there are all those little pictures from the predella giving the narratives of the holy lives and Christ's crucifixion. Precisely because they were so lively, they have been scattered around the museums and the private collections of the world as small pieces of art in their own right. They're not, of course. They were part of the whole, and a part that only the priest saw, reminding him of the saintly deeds of the martyr to which the altar was dedicated. They also clearly intrigue the conservators as they try and tell, from structure and style, just where they fitted into the sequence of the altarpieces.
In the end the exhibition is more fascinating for what it tells you about presentation than meaning. It is almost impossible now to know just what feelings they aroused at the time, certainly for today's secular visitor. But, as an insight into just how these works might have looked before they were broken up or reworked, this exhibition is a must – before or after you see the masterpieces in the main galleries.
Devotion by Design: Italian Altarpieces Before 1500, National Gallery, London (020 7747 2885; www.nationalgallery.org.uk) until 2 October