Modern city planning has seen many different schemes for how a city should look. The most commonly seen pattern is the grid, favoured by the Romans, almost a rule in parts of the Americas, and used for thousands of years in China. Derry was the first planned city in Ireland, begun in 1613, with the walls being completed five years later. The central diamond within a walled city with four gates was thought to be a good design for defence. The grid pattern was widely copied in the colonies of British North America.
The Ancient Greeks often gave their colonies around the Mediterranean a grid plan. One of the best examples is the city of Priene. This city had different specialized districts, much as is seen in modern city planning today. Fifteen centuries earlier, the Indus Valley Civilization was using grids in such cities as Mohenjo-Daro. In medieval times there was evidence of a preference for linear planning. Good examples are the cities established by various rulers in the south of France and city expansions in old Dutch and Flemish cities.
Grid plans were popular among planners in the 19th century, particularly after the redesign of Paris. They cut through the meandering, organic streets that followed old paths. The United States imposed grid plans in new territories and towns, as the American West was rapidly established, in places such as Salt Lake City and San Francisco.
Other forms may include a radial structure, in which main roads converge on a central point. This was often a historic form, the effect of successive growth over long time with concentric traces of town walls and citadels. In more recent history, such forms were supplemented by ring-roads that take traffic around the outskirts of a town. Many Dutch cities are structured this way: a central square surrounded by concentric canals. Every city expansion would imply a new circle (canals + town walls). In cities such as Amsterdam and Haarlem, and Moscow, this pattern is still clearly visible.
