Walls-think of the great walls of history: The Chinese Wall, the Wailing Wall, the Berlin Wall, the Walls of Jericho and the wall on which the handwriting appeared to Belshazzar. Think of Zechariah's wall of fire and of the waters of the Red Sea which 'were a wall to them on their right hand and on their left'. Remember the walls of literature: Sartre's wall of execution, Shakespeare's 'sweet and lovely wall', and the wall raised in front of your (and the unfortunate Fortunato's) eyes in Poe's poem 'Cask of Amontillado'. And think of the expressions: up against the wall, to go over the wall, wall-to-wall; think of wallflowers, of walls of silence and of separation, of stonewalling and of Stonewall Jackson, of invisible walls and of the hole-in-the-wall. An exhaustive list of famous walls, religious and other, would, of course, be impossible to compile. Walls have inspired cinema makers, science fiction writers, poets, playwrights, painters and muralists. In one of the literary sources in Ref. 2 (F. Dostoyevsky) the protagonist's adversaries try to convince him about the immutability of the laws of nature: 'A wall, you see, is a wall ... and so on, and so on.' But is it? It is my thesis that, in addition to their versatile physical functions, walls possess an immense measure of signification and that these two realms-the concrete and the symbolicinteract with each other. Take first the multiple significance of walls for the visual arts, in whose history walls have a place of distinction not merely because pictures are hung on them. Since prehistorical times, unadorned walls have attracted adornment, giving rise to such different modes and media of artistic expression as cave drawings, frescoes, murals and street art, as well as reliefs, tapestry, wall hangings and posters [3]. The motivation behind these expressions has a wide range, from the homeopathic magic presumably practised by the Cro-Magnon artists of the Altamira caves in Spain, through the religious murals of the Neolithic culture in Catal Hiuyiik in Anatolia, to the secular, decorative tapestries of the Gothic era and to the political murals and posters of the present age. Next consider architecture, where walls may be said to define the art itself, for architecture is the art of bounding, the provision of multiple volumes that are separated from one another and from their surroundings by walls [4]. And yet the function of walls in architecture has drastically changed throughout the ages: the Romanesque tradition of heavy, structurally indispensable walls has gradually given way to the decorative facades of the Gothic period with their buttress-supported vaults.