Fifteen years before Christopher Alexanders deservedly celebrated books on patterns in architecture and at least twenty before anyone had heard of design patterns in software, Jane Jacobs, in her 1965 book The Death and Life of Great American Cities, laid out an urban pattern language. Her patterns include short blocks, residence and work mixed together (also one of Alexanders patterns), aged buildings (interspersed with newer ones to promote diversity of use), wide sidewalks (to give children safe, because supervised, areas to play), and high dwelling, as distinct from population, density. City neighborhoods that have these patterns are alive; neighborhoods that lack them are dead. But as Jacobs points out, the urban planners like Corbusier, Ebenezer Howard, Lewis Mumford, and the other prophets of what she calls Garden City opposed these patterns at every turn. To see their legacy in Manhattan, look at Stuyvesant Town, or any housing project. Essentially they hated cities and proposed to fix them by making them as uncitylike as possible.

The power of Jacobs book lies in its specificity. To get some idea of her method, consider one of her patterns, high dwelling density, in some detail. She first takes pains to distinguish dwelling from population density:

The Garden City planners and their disciples looked at slums which had both many dwelling units on the land (high densities) and too many people within individual dwellings (overcrowding), and failed to make any distinction between the fact of overcrowded rooms and the entirely different fact of densely built up land. They hated both equally, in any case, and coupled them like ham and eggs, so that to this day housers and planners pop out the phrase as if it were one word, “highdensityandovercrowding.”

But here the planners run into difficulty: often the most successful areas of a city, because they have high dwelling densities, have high population densities as well, but without overcrowding. And the opposite is also true: areas with low population densities, like Bedford-Stuyvesant, are often full of overcrowded dwellings.

Having isolated the real question, dwelling density, Jacobs proceeds from low to high to see what works and why. Six to ten dwellings per acre, yielding lots of 70 by 100 feet or so, can succeed in a suburb. Ten to twenty dwellings per acre tends to look like semi-detached row houses, like parts of Queens.

These arrangements, Jacobs writes, although they are apt to be dull, can be viable and safe if they are secluded from city life…They will not generate city liveliness or public lifetheir populations are too thinnor will they help maintain sidewalk safety. But there may be no need for them to do so.

Above 20 dwellings per acre, however, youre in a city:

From this point on, a city settlement needs city vitality and city diversity. Unfortunately, however, densities high enough to bring with them innate city problems are not by any means necessarily high enough to do their share in producing city liveliness, safety, convenience and interest. And so, between the point where semisuburban character and function are lost, and the point at which lively diversity and public life can arise, lies a range of big-city densities that I shall call “in-between” densities. They are fit neither for suburban life nor for city life. They are fit, generally, for nothing but trouble.

Successful urban areas typically have dwelling densities of at least 100 to the acre, and sometimes many more. And yet the urban planners consistently recommended population densities of about 100, which meant dwelling densities of about 25 to 50the exact recipe for gray areas and blight.

It seems incredible now, but in the early 1960s, Robert Moses, New Yorks Parks Commissioner and public works czar, planned a series of east-west expressways through Manhattan, beginning with Greenwich Village. One of the best passages in Jacobs book is a description of the intricate street life, the staggered comings and goings, on her block on Hudson Street, and a more wanton destruction of this sort of life could hardly be imagined. She remarks in her book that There are only two ultimate public powers in shaping and running American cities: votes and money. To sound nicer, we may call these public opinion and disbursement of funds, but they are still votes and money. Lacking money but having votes, Jacobs put her principles into practice, organized her neighbors against the expressway and defeated Moses, who at the time was at the height of his powers. Moses lost his momentum, and none of his proposed cross-Manhattan expressways came to pass. We should thank her by reading her.