The distinctive characteristic of Italian Gothic is the use of polychrome decoration, both externally as marble veneer on the brick façade and also internally where the arches are often made of alternating black and white segments, and where the columns may be painted red, the walls decorated with frescoes and the apse with mosaic. The plan is usually regular and symmetrical, Italian cathedrals have few and widely spaced columns. The proportions are generally mathematically equilibrated, based on the square and the concept of "armonìa", and except in Venice where they loved flamboyant arches, the arches are almost always equilateral. Colours and moldings define the architectural units rather than blending them. Italian cathedral façades are often polychrome and may include mosaics in the lunettes over the doors. The façades have projecting open porches and occular or wheel windows rather than roses, and do not usually have a tower. The crossing is usually surmounted by a dome. There is often a free-standing tower and baptistry. The eastern end usually has an apse of comparatively low projection. The windows are not as large as in northern Europe and, although stained glass windows are often found, the favourite narrative medium for the interior is the fresco.