The floor of St. Mark’s church is an actual marble carpet spread over no less than 2099 square metres.
Following the tenets of Byzantine religious architecture, St. Mark’s too complies with the principle of bipartition into earthly zone (floor and walls) and celestial part (vaulted ceilings and cupolas). Purpose and function are underlined by the different materials used to cover the masonry. The upper part of the building has a strikingly celestial and metaphysical connotation due to the light produced by the tesserae in coloured glass or gold leaf, symbolising the light of paradise, whereas the lower zone underlines earthliness with the solidity of the marbles of the walls (rich in colours, but dull ones, and in geometrical signs) and of the floor.

The floor of St. Mark’s includes opus sectile (obtained by setting out pieces of different coloured marble to create the most varied geometrical forms) and opus tessellatum (obtained with tiny pieces of marble or glass used to create floral motifs or animal figures), with a clear prevalence of the former technique over the latter. Both techniques have their origins in antiquity, as documented by Varro, Vitruvius and Pliny. Coexistence of the two in St. Marks testifies to the great wealth of the dukedom: it not only bought up highly precious marbles but also secured a workforce of craftsmen who, in all probability, were brought to Venice fromConstantinople or Byzantine Greece, as were the architects and mosaicists.
The overall floor consists of various panels, of different sizes, with geometrical and figured motifs. Certain surfaces in well illuminated zones, such as the areas beneath the Pentecost and Ascension cupolas, are faced with great slabs of Greek Proconnesio marble, one of the first marbles to be cut into slabs.

The geometrical organisation is regular and the positioning observes the principles of symmetry where possible.
The nave has a sequence of larger fairly linear decorations. Near the entrance there is a large herring-bone decorated rectangle which includes a smaller central rectangle with similar decoration. Farther on towards the presbytery there is a second large rectangle that contains two rows of rhombuses and polychrome rote(“wheels”) punctuated by four squares alternated with three rhombuses.

The arms of the transept contain two squares: the northern includes decorations of five large Byzantine rote and four small ones interposed between them; in the southern, a lozenge-design carpet with frame is followed towards the south by four Byzantine rote.
In this rigorously geometrical scheme there are also, on the margins, symbolic animals and floral elements: outstanding for their chromatic preciousness and refined execution are two pairs of peacocks in the small southern or right aisle which have been preserved almost intact.

Along the arc of the Upper Adriatic there are many examples of mosaic floors. But the floor of St. Mark’s stands out for its grandeur and the preciousness and rarity of the eastern, western and North African marbles employed, as well as for the splendour of the enamels and the variety of scenes drawn from symbolism and mediaeval literature, or inspired by oriental and western textiles.
The whole is based on an iconographic programme which is very complex for us but which could be more easily intuited by the people of the Middle Ages.