The city center was constructed on the old colonial basis. It features a grid system of tiny streets built out at right angles. This rectilinear pattern extends across more than 20 square blocks, a region that established the city's boundaries until the late nineteenth century. The central plaza is surrounded by buildings in this style, including the government palace and the cathedral. Each block has an average area of about 100 yards squared, but can range up to three times as large or small. The smallest are used for residential homes, while the largest are used for commercial or public purposes. The majority of city centers around the world are built according to the same basic plan: a grid system of streets lined with office buildings, shops, and restaurants. In fact, many cities were originally founded around a central market place served by a series of roads leading away from it. As they grew larger, these road networks became important routes for trade and transportation, so they were gradually replaced by orthogonal (straight) streets when the new cities were planned out. In Europe, the early modern period saw the emergence of downtown areas. But it was not until the mid-nineteenth century that they began to take shape in a significant way. Central Paris was rebuilt following the 1852 earthquake, replacing long stretches of urban sprawl with a grid of wide boulevards flanked by symmetrical rows of tall buildings.