What is a block? Such a simple question does not seem to warrant even a blog post. After all, it is a term that has been used frequently by pretty much everyone—from Jane Jacobs to Jennifer Lopez. And block size is one of the first attributes sought by urbanists when studying a town or city. However, while the term is used widely there does not seem to be a consensus for the actual definition of a block. How does one actually measure a block? That is the question under investigation.
Typing “What is a block?” into Google reveals a few attempts at answers. Wikipedia calls it an “informal unit of distance.” Some respondents at Yahoo! Answers say it is “the distance from one intersection to the next” while other respondents from the UK proclaim that they do not even use the term because all of their blocks are too irregular. And still others have stated that block size should be measured at the street curb or sidewalk. All of these answers, however, miss a critical aspect of what a “block” really means.
A block arises from the fact that cities are composed of two (and only two) types of property: public and private. This critical distinction between public and private property has been the basis of urban design since the concept of “street” first appeared in ancient Mesopotamia. In these early cities, public streets began to give access to private property. With this, people started to distinguish between what is mine, yours, and ours.
The specific way that public and private property is used is a function of ownership, regulation, economy, and functional necessity. Public streets can be gravel roads or grand boulevards. Private houses can form a street wall or fall behind setbacks. Regardless of these internal differences, all properties have one thing in common: they share their boundary lines. These boundary lines are in turn locked into place by law and come to act as the binding agent of urbanism. For towns and cities alike, this legal structure translates directly into physical structure. The boundary line acts as both the legal framework and physical formwork. In other words, urban development–from Paris, TX to Paris, France–simply follows property lines.
All of that being said, the definition of a block should be based on the legal structure of urbanism. Therefore, a block is legally defined as private property surrounded by public rights-of-way. By this definition, a block is one of the two fundamental units of urbanism (alongside the right-of-way) reflecting the two types of property (private and public, respectively). An example of a block is shown below. Also keep in mind that a public park is effectively absorbed into the public right-of-way. Because of this, no boundary line is drawn to separate the park from the right-of-way. This is often illustrated in historical maps as well as Nolli’s map of Rome.
Block size is then measured from right-of-way to right-of-way. It should not be measured from street centerlines or curbs or sidewalks. These elements of urbanism are temporary and change constantly over time. Boundary lines, on the other hand, are as close to constant as urbanism can get. The blocks in Manhattan have not shifted an inch for over 200 years even though their internal boundaries and land uses have all changed dramatically in that time. While cities may change internally, their right-of-way lines predominantly do not. Boundary lines over time resist change and can outlast any mere building or government. It is a bootstrapping attribute of the power of legal subdivision: boundary lines have staying power because they have staying power.
This definition of block size allows a constant number to be assigned to a city’s urbanism. In 1811, the block at Madison Avenue and 71st Street in Manhattan (pictured below) had the exact same dimension then as it does now even though the block’s land use has changed from a farm to skyscrapers. Once property lines are established, the lineage of ownership is essentially locked into its original geometry.
This definition also takes into account the absurdities found in suburbia. In the land of the cul-de-sac, block perimeters can easily reach into the 10’s of miles. In Alpharetta, Georgia (below), this one measures about 12 miles.
To conclude, by referencing the legal basis of city planning block size can be used as a universal and constant measure of urbanism. The block, being private property surrounded by public rights-of-way, is one of the fundamental units of urbanism. And with that, the consensus between Jane Jacobs and J-Lo has been restored.