Thin, flat slab or block used structurally or decoratively in building. Tiles traditionally have been made of glazed or unglazed fired clay, but modern tiles are also made of plastic, glass, asphalt, and even cork. Ceramic tiles, used for walls, floors, and countertops, are usually machine-pressed, made of fine clays, and very hard. Quarry tiles (used for flooring) and terra-cotta, made of natural clays, are less hard and more porous but very popular for economic and aesthetic reasons. Structural tile, made of fired clay, is a hollow tile containing parallel cells or cores and is used for building partitions. Roof tiles of baked clay and of marble were used in ancient Greece. Tiles came to be widely used in Islamic architecture. Multicoloured, glazed tiles were common in Spain from an early period (seeazulejo), and from there spread to Portugal and Latin America. By the 15th century, tilework was used widely in northern Europe; blue-painted tiles from Delft, Holland, were especially renowned. Modern clay roofing tiles may be flat or curved; in the Mediterranean countries, S-shaped tiles (pantiles), laid with alternate convex and concave surfaces uppermost, are common. Modern wall tiles may be highly glazed and semivitreous.