A city block is the smallest area that is surrounded by streets. City blocks are the space for buildings within the street pattern of a city, and form the basic unit of a city's urban fabric. City blocks may be subdivided into any number of smaller land lots usually in private ownership, though in some cases, it may be other forms of tenure. City blocks are usually built-up to varying degrees and thus form the physical containers or 'streetwalls' of public space. Most cities are composed of a greater or lesser variety of sizes and shapes of urban block. For example, many pre-industrial cores of cities in Europe, Asia and the Middle-east tend to have irregularly shaped street patterns and urban blocks, while cities based on grids have much more regular arrangements.
In most cities of the world that were planned, rather than developing gradually over a long period of time, streets are typically laid out on a grid plan, so that city blocks are square or rectangular. Using the perimeter block development principle, city blocks are developed so that buildings are located along the perimeter of the block, with entrances facing the street, and semi-private courtyards in the rear of the buildings. This arrangement is intended to provide good social interaction among people.
Since the spacing of streets in grid plans varies so widely among cities, or even within cities, it is difficult to generalize about the size of a city block. However, as reference points, the standard square blocks of Portland, Houston, and Sacramento are 264 by 264 feet (80 m × 80 m), 330 by 330 feet (100 m × 100 m), and 410 by 410 feet (120 m × 120 m) respectively (to the street center line). Oblong blocks range considerably in width and length. The standard block in Manhattan is about 264 by 900 feet (80 m × 274 m); and in some U.S. cities standard blocks are as wide as 660 feet (200 m). The blocks in Calgary, Canada, are 330 by 560 feet (100 m × 170 m), while those in Edmonton, Canada are 197 by 560 feet (60 m × 171 m). The blocks in central Melbourne, Australia, are 330 by 660 feet (100 m × 200 m), formed by splitting the square blocks in an original grid with a narrow street down the middle. In Chicago, Illinois and Minneapolis, Minnesota, a typical city block is 660 by 330 feet (200 m × 100 m) (w × h), meaning that 16 east-west blocks or 8 north-south blocks measure one mile.
Many world cities have grown by accretion over time rather than being planned from the outset. For this reason, a regular pattern of even, square or rectangular city blocks is not so common among European cities, for example. An exception is represented by those cities that were founded as Roman military settlements, and that often preserve the original grid layout around two main orthogonal axes. One notable example is Turin, Italy. Following the example of Philadelphia, New York City adopted the Commissioners' Plan of 1811 for a more extensive grid plan. By the middle of the 20th century, the adoption of the uniform, rectilinear block subsides almost completely and more picturesque layouts prevailed, with random sized and either curvilinear or non-orthogonal blocks and corresponding street patterns.