# Chapter 6: The Four Essential Rules After measuring hundreds of great places, analyzing thousands of dimensions, and discovering the universal patterns that make urbanism work, we arrive at something surprisingly simple: four rules. Just four—not forty, not four hundred. Four rules that capture perhaps 90% of what makes the difference between places that thrive for centuries and places that struggle from day one. These aren't arbitrary impositions or theoretical ideals. They're distillations of what already works, refined to essentials. Think of these rules as the DNA of good urbanism. DNA is remarkably simple—just four base pairs combining in endless variations—yet it contains all the instructions needed to build complex organisms. Similarly, these four rules provide the basic instructions for building adaptable, walkable, lasting places while allowing infinite variety in their expression. ## The Nature of Rules Before diving into specifics, let's be clear about what these rules are and aren't. They're not rigid commandments that guarantee success if followed blindly. Cities are too complex, too human for that kind of mechanical certainty. Instead, think of them as fundamental principles—like the rules of grammar that enable clear communication while allowing for poetry, or the rules of music that create harmony while permitting jazz. They're also not meant to be followed 100% of the time. Even the best neighborhoods have exceptions. Charleston's South of Broad district, one of America's most beloved neighborhoods, has blocks that violate Rule #1. Commonwealth Avenue in Boston's Back Bay, one of the most beautiful streets in America, falls well outside the suggested range in Rule #3. The goal isn't perfection but predominance—follow the rules most of the time and you'll create a framework for good urbanism. Most importantly, these rules address only the permanent framework—the constitutional order of streets and blocks. They don't dictate architecture, uses, or activities. They create the stage, not the play. ## Rule #1: Block Size > [!Tip] **Blocks should have sides greater than 200 feet and less than 600 feet, with a perimeter less than 1,800 feet.** Of all the rules, this one matters most. Get block size right and you're 80% of the way to a foundation for good urbanism. Get it wrong and almost nothing else can save you. ### The Discovery in the Data When we compare the median-sized blocks of various neighborhoods, something striking emerges. At first glance, some blocks are a little more angular than others, some have alleys while some don't, but overall they are strikingly similar. They are all relatively the same size, relatively the same shape. Notably, in some cases they are indistinguishable: the Garden District of New Orleans and downtown Madison, Wisconsin, utilize the same block. Charleston and Vienna are barely distinguishable from one another. A worldwide pattern is already evident. ![[block-compare-figure-ground.png]] As we discovered in Chapter 5, when neighborhoods from around the world are plotted by their block dimensions, they cluster tightly together—not scattered randomly across the graph, but huddled in one concentrated area. This astonishing consensus, the product of millions of decisions over centuries, gives us our first and most important rule. ### Why These Dimensions? The power of proper block size lies in its relationship to the things we actually build. The vast majority of human activities—living, working, shopping, learning—fit comfortably on lots ranging from 90 to 120 feet deep. A house with a yard needs about 90 feet. An office building could use 120 feet. When blocks are 200-400 feet deep (the most common range in great places), they accommodate two tiers of these human-scaled lots placed back-to-back. Every lot faces a street, maximizing access and visibility. The math is simple but powerful: a 300-foot-wide block splits perfectly into two 150-foot-deep lots, or two 120-foot lots with a 20-foot alley and two trays of parking between them, or various other useful configurations. If we break it down between each side of a block, the acceptable ranges get even tighter. The average short side range is only 200 feet to 400 feet; the average long side range is larger: 300 feet to 700 feet. The reason for this is that the short side tends to be a function of lot depth. Because blocks are typically composed of two lot tiers and because lot depths tend to fall in the 100 to 200 foot range, this narrows the overall range of block width. Block length, however, is a function of lot width and can change simply by appending more lots side by side. In this case, the block length can easily expand without sacrificing lot efficiency. This is why block length falls within a higher range. But when blocks grow beyond 600 feet, problems begin to cascade. Salt Lake City's 660-foot blocks create lots that are 330 feet deep—far more than most uses require. Property owners inevitably sell off the backs of their lots, creating an inner ring of development that doesn't face any street. These interior properties, lacking street frontage, struggle to attract investment or contribute to city life. ![[salt-lake-large-block.png]] ### The Perimeter Principle The perimeter limit of 1,800 feet prevents another kind of error. While a block might have acceptable dimensions on each side—say, 600 feet—if all four sides hit that maximum, you'd get a 2,400-foot perimeter. That's Salt Lake City territory, far too large for walkable urbanism or efficient land utilization. The perimeter rule ensures that even if one or two sides of a block reach toward 600 feet, the other sides must be shorter to keep the total perimeter under 1,800 feet. This prevents the creation of superblocks by stealth—blocks that technically meet the side-length requirements but still destroy walkability through their sheer size. Think of it as a failsafe. A block that's 300 by 600 feet has a perimeter of 1,800 feet—just within limits. But a block that's 600 by 600 feet has a perimeter of 2,400 feet—well beyond what works for human-scale urbanism. The perimeter rule catches these oversized blocks that individual side measurements might miss. ### A Universal Pattern, Not a Density Determinant It is a common misconception to assume that small blocks imply high population density and big blocks imply low population density. As it turns out, block size has little relationship to population. In our analysis, populations span a wide spectrum, ranging from 488 people in Paragonah, Utah, to 2.2 million people in Paris, France. Yet among all the cities studied, their block sizes fall within an incredibly tight range, with a relative standard deviation of only 28%. This bears repeating from our earlier chapter because it directly refutes one of the most common objections to proper block sizes—that they're only suitable for dense cities. The data proves otherwise: human-scaled blocks work everywhere, from tiny towns to global capitals. What this reveals is a universal urban form: **all these cities represent the spectrum of everything ever built on the planet, and yet they're all operating within the same relative block size.** This universality extends down to the individual lot. A standard 60-foot by 120-foot lot is a surprisingly versatile unit of land. That lot could accommodate everything from a house to a parking lot to a skyscraper. While the heights and land use intensities vary, the critical point is that all these uses occupy the same footprint. In plan view, a house with a yard can take up just as much space as a 30-story skyscraper. ![[lot-uses.png]] ### The Multiplication Effect Here's what makes block size so critical: its effects multiply throughout the city. Blocks that are too large don't just create one problem—it creates cascading problems at every scale: - **For individual properties**: Lots become too deep, wasting land or creating awkward subdivisions that don't face streets - **For neighborhoods**: Fewer blocks mean fewer streets, reducing both connectivity and the amount of "neighborhood" available per unit area. - **For cities**: The pattern becomes locked in by property lines, nearly impossible to fix once built Alpharetta, Georgia, demonstrates this multiplication of error. When blocks are measured in thousands of feet rather than hundreds, you create a landscape where walking becomes impossible, every trip requires a car, traffic concentrates on few arterial roads, there's less "_there_, there", and change is nearly impossible because the pattern is locked in by development. ### The Flexibility Within Limits Yet within the 200-600 foot range, considerable flexibility exists. A 280-by-520-foot block—a common unit found in our study—can accommodate dramatically different uses: - In Chicago: Condo towers, townhouses, and bungalows - In Amsterdam: Canal houses and palaces - In Buenos Aires: Apartments and markets - In suburban America: A grocery store with parking - Anywhere: A school, a park, offices, or 16 single-family houses Same dimensions, dramatically different uses, all working successfully. The dimensional DNA remains constant while the expression varies infinitely. This is the power of getting block size right—it creates a framework that can adapt to whatever the future brings while maintaining the human scale that makes places livable. ## Rule #2: Block Geometry > [!Tip] **Blocks should be composed of straight line segments with preference given to rectangles. A long side to short side ratio of 1.5:1 or greater is also preferred.** This might seem mundane, even obvious. Of course blocks should be rectangular—what else would they be? But drive through any suburb built in the last fifty years and you'll see: curved blocks, irregular blocks, triangular blocks, blocks shaped like amoebas. The abandonment of rectilinear geometry is one of the clearest markers distinguishing traditional urbanism from its modern substitute. ### The Rectangle Rules Rectangles dominate human construction for excellent reasons. Look around you right now. The text for this book is organized in rectangular columns. The screen you might be reading it on is rectangular. The room you're in probably has rectangular walls, rectangular doors, rectangular windows. Your desk, your bed, your refrigerator—rectangles are everywhere. This isn't cultural bias or lack of imagination. It's functional optimization refined over millennia. As the Manhattan commissioners noted in 1811, _"straight-sided and right-angled houses are the most cheap to build and the most convenient to live in."_ This was true in ancient Mesopotamia, true in 1811, and true today. **When blocks are rectangular, this efficiency propagates throughout the urban fabric. Rectangular lots fit efficiently within rectangular blocks. Rectangular buildings fit efficiently on rectangular lots. Rectangular rooms fit efficiently within rectangular buildings. Each scale nests within the next like Russian dolls, minimizing waste and maximizing utility.** ### The Cost of Curves But surely curves are more natural, more organic, more interesting? Let's test that assumption empirically. Take four blocks, each with the same area—three acres—but different geometries: a rectangle, an irregular angular block, a triangle, and an oval. Now try to subdivide them into standard 60-by-120-foot lots: - Rectangle: 16 lots fit comfortably - Irregular: 15 lots, with awkward leftover spaces - Triangle: 14 lots, with unusable sharp corners - Oval: 12 lots, with wasted curved edges ![[geometry-yield-table.jpg]] The geometric efficiency loss is 25% between the rectangle and the oval. Over a 160-acre development, that's 40 acres of underutilized land—worth millions of dollars, representing hundreds of homes that can't be built, thousands of people who can't be housed. ![[geometry-yield.png]] But the inefficiency goes beyond simple lot yield. Curved blocks create: - Needlessly complex property boundaries requiring more surveying - Irregular building footprints requiring custom designs - Higher construction costs for curved walls and angled connections - Difficult furniture placement in wedge-shaped rooms - Confused circulation patterns - Higher infrastructure costs for curved utilities - Greater difficulty in creating "street walls" with building facades (the "face" of the city). Remember: these blocks will last for centuries, possibly millennia. Every inefficiency gets multiplied by time. A 25% efficiency loss compounded over 200 years represents enormous waste and inflexibility. ### The Ratio Rationale Why prefer blocks with a length-to-width ratio of 1.5:1 or greater? The answer emerges from both precedent and practical subdivision. First, simple observation: when we analyzed hundreds of successful neighborhoods, rectangular blocks dominated. Not squares, not irregular shapes—rectangles. This consistency across cultures and centuries suggests something fundamental about how rectangles work for human habitation. Second, the challenge of corners. Square blocks create a subdivision puzzle that even Thomas Jefferson couldn't solve elegantly. When he designed his ideal city with square blocks, the corner lots became awkward—too small when divided equally, too large when combined. The geometry fights against efficient subdivision. While some cities like Barcelona make square blocks work through careful design, they require more conscious planning to avoid dead corners and wasted space. Third, and most importantly, rectangular blocks are easier to line with active frontage on all sides. Here's why: imagine a block that's 300 feet wide by 450 feet long. You can cap each short end with a row of lots facing outward—say, six 100-foot-deep lots. Then along each long side, you can fill the middle portion with another row of five lots, also facing outward. The result? A block completely lined with front doors, front yards, and active faces. No dead zones, no confused orientations, no wasted corners. This is much harder with a square block. A 375-by-375-foot square (same area as our rectangle) creates corner conflicts. If lots on all four sides are the same depth, the corners overlap. If you avoid overlap by making corner lots smaller, they become less desirable. If you make them larger, you waste land. The square fights against simple subdivision. The elongated rectangle solves this elegantly. The 1.5:1 ratio (or greater) ensures enough length to create a clear "grain" to the block—short ends and long sides that naturally accommodate different lot arrangements. This isn't about predetermining uses or creating hierarchy. It's about maximizing flexibility for whatever future uses emerge. A 300-by-600-foot block (2:1 ratio) can be subdivided dozens of ways: - Twenty 60-by-150-foot lots for single-family homes - Forty 30-by-150-foot lots for townhouses - Mixed sizes for varied housing types - Large parcels for institutional uses - Any combination as needs change over time Each configuration maintains active frontage on all sides. Each works with standard dimensions of construction. Each can evolve into something else as the city changes. The elongated rectangle offers maximum flexibility within a simple, efficient form. ### The Grid Paradox Critics often dismiss rectangular blocks as boring, monotonous, uncreative. "It's just a grid," they say, as if this were self-evidently bad. Yet the places people pay premium prices to live and love to visit—Manhattan, San Francisco, Back Bay Boston, Georgetown, Savannah, and hundreds of great railroad towns across America—are mostly grids. The function of a street network is to connect people and places efficiently while creating developable land. The rectangular grid does this better than any other geometry. That's why it appears independently in ancient China, classical Greece, Roman colonies, medieval new towns, and American cities. It's not cultural imperialism—it's convergent evolution toward optimal form. The supposed monotony of grids also misunderstands how cities actually develop. The grid provides neutral infrastructure. What gets built on it creates variety. Walk through Greenwich Village or Barcelona's Eixample—both rigid grids—and try to feel bored. The regular geometry liberates architecture and activity to create interest. The boring suburbs with their curved streets and irregular blocks? They're boring partly because the geometry tries too hard to be interesting, leaving no room for life to create actual interest. ## Rule #3: Rights-of-Way > [!Tip] **Most rights-of-way should have a width between 40 feet and 80 feet. A limited number of rights-of-way may be larger or smaller.** Streets are not just about traffic. If you think of streets primarily as channels for moving vehicles, you've already lost the plot. Streets are the public rooms of the city, the addresses of buildings, the routes of infrastructure, the shapers of blocks, the creators of value. Movement is just one function among many. ### The Universal Width When we measured hundreds of successful neighborhoods, one width appeared more than any other: 60 feet. From the grand boulevards of Paris to the intimate streets of Charleston, from dense Manhattan to sprawling suburban towns, 60 feet emerges again and again. Why? Because 60 feet is the Swiss Army knife of street widths—versatile enough to handle almost any condition: - **Rural**: Two lanes with generous shoulders and drainage swales - **Suburban**: Two lanes, parking on both sides, sidewalks, and tree lawns - **Urban**: Multiple configurations—from four narrow lanes to two lanes with protected bike lanes and wide sidewalks - **Commercial**: Wide sidewalks for cafes, street trees, on-street parking, and delivery zones The same 60-foot right-of-way that starts as a country road can evolve into an urban thoroughfare without changing its fundamental geometry. The boundary lines remain constant while the use within them adapts to changing needs. ![[381st-ave-iowa.png]] ![[bowen-street-providence.png]] ![[41st-street-new-york.png]] While 60 feet may be the workhorse, the full range of 40 to 80 feet provides necessary variety (from intimacy to grandeur) while being a "goldilocks zone" for future adaptability. ### The Distribution Principle The rule says "most" rights-of-way should fall within this range, acknowledging that special conditions require special responses. A major, "city-scaled" thoroughfare might need 100 feet or more. A pedestrian alley might work at 20 feet or less. A highway is a different creature entirely, playing by different rules. But these exceptions should be exceptional. When every street is extra-wide "just in case," you don't get a resilient network—you get a wasteland of asphalt that spreads destinations beyond walking distance, costs fortune to maintain, and deadens city life. The best networks follow a power law: many smaller streets, fewer medium streets, very few large streets. Like capillaries, arteries, and veins in the body, each scale serves a purpose, but the finest grain does most of the work. ![[ROW-tally.png]] ## Rule #4: Alleys > [!Tip] **Alleys should be between 10 and 20 feet in width and be present in most, if not all, blocks.** If streets are the public faces of blocks, alleys are their workhorses—unglamorous but essential. This one is often ignored in modern development. The consequences of this neglect are visible in urban centers and suburbs alike: multiple curb cuts along streets, garbage cans crowding driveways, utility lines marring front yards, garage doors dominating street facades, delivery trucks blocking traffic. ### The Hidden Hero Commonwealth Avenue in Boston ranks among America's most beautiful streets—a linear park flanked by elegant townhouses, tree-lined and gracious, with a linear park punctuated with monuments. But this beauty depends entirely on the narrow alleys threading between the blocks. These alleys absorb all the necessary ugliness of urban life: dumpsters, utility poles, parking, service access, mechanical equipment. Without them, Commonwealth Avenue would be just another street cluttered with the detritus of daily life. This is the alley's gift: it liberates the street to be beautiful. When you have two faces to every lot—front and back—you can present your best face to the public while handling necessity in private. It's the urban equivalent of a mullet: clean in front, messy in back. ![[alley-diagram-2.png]] ![[alley-diagram-1.png]] ### The Multiplication of Access Alleys double the access points to every lot. This might seem like a small thing, but the ramifications ripple through every aspect of urban life: **For residential properties**: The garage goes in back, freeing the front for porches, gardens, and social life. Trash collection happens out of sight. Utilities enter from the rear, eliminating front-yard power lines. **For commercial properties**: Deliveries occur in back, keeping sidewalks clear for customers. Dumpsters hide behind buildings instead of occupying precious frontage. **For the street network**: Every alley adds to total connectivity, providing alternative routes for local trips. Service vehicles use alleys instead of blocking travel lanes. The finer grain of circulation reduces congestion on main streets. ### The Consolidation Effect Without alleys, every lot needs its own curb cut—a driveway piercing the sidewalk. On a typical block face of 400 feet with 50-foot lots, that's sixteen driveways, sixteen conflict points between cars and pedestrians, sixteen breaks in the comfortable walking rhythm. With an alley, those sixteen driveways consolidate to two alley entrances—one at each end. The sidewalk runs uninterrupted. Street trees grow without interference. Parallel parking remains continuous. The pedestrian experience transforms from obstacle course to pleasant stroll. ### The Dimensional Sweet Spot Why 10 to 20 feet? Because precedent said so and because alleys need to accommodate specific functions: **10-12 feet**: The minimum for one-way traffic with occasional passing. Garbage trucks can navigate. Utilities fit. This tight dimension ensures alleys remain secondary, not attractive cut-throughs for speeding traffic. **15-18 feet**: The comfort zone. Two cars can pass. Delivery trucks maneuver easily. There's room for utility poles without blocking access. Some cities permit accessory dwelling units (ADUs) along alleys this wide. **20 feet**: The maximum before alleys start becoming streets. Beyond this, the expectation of sidewalks arises, speeds increase, and the alley loses its service character. ### The Manhattan Mistake No city demonstrates the cost of missing alleys more clearly than Manhattan. When the commissioners laid out their famous 1811 grid, they made one catastrophic error: they eliminated alleys to maximize developable land. For over 200 years, New Yorkers have paid for this mistake. Every day is trash day somewhere in Manhattan. Plastic bags pile on sidewalks, creating obstacle courses for pedestrians, leaching smells, attracting rats, destroying the elegance of otherwise beautiful streets. Deliveries block traffic lanes because there's nowhere else to stop. Every building needs its own loading dock carved from precious ground-floor retail space. The bitter irony is that the commissioners saved perhaps 8% of land by eliminating alleys. But the loss of efficiency, the daily friction, the aesthetic degradation, the sheer unpleasantness imposed on millions of people over centuries—the cost defies easy calculation. To fix it now would require assembling slivers of the most expensive real estate on Earth. So the garbage remains, piled on the sidewalks of even the richest neighborhoods, a daily reminder that some planning mistakes are forever. ![[no-alleys-manhattan.png]] ## The Rules as System These four rules work together as an integrated system. Proper block sizes create the need for appropriate street widths. Rectangular geometry makes both efficient. Alleys make it all work smoothly. You can't cherry-pick—using just one or two rules while ignoring others didn't work for the precedent and won't work for you. Think of it like baking. Flour, water, yeast, and salt are simple ingredients. Use them in proper proportions and you get bread—infinitely variable in form but recognizable as the staff of life. Omit the yeast and you get hardtack. Too much salt and it's inedible. Wrong proportions and you get paste. The magic isn't in any single ingredient but in their relationship. Similarly, these rules create a framework for urbanism that's proved successful across cultures and centuries. Within this framework, infinite variation is possible—from the intimacy of Greenwich Village to the grandeur of Paris, from the charm of Savannah to the bustle of Barcelona. The rules don't create monotony; they create possibility. ## Using the Rules How should planners, developers, and citizens use these rules? Not as rigid commandments but as default settings. Start with the assumption that blocks will be 200-600 feet, mostly rectangular, served by 40-80 foot streets with 10-20 foot alleys. Then, if special conditions require variation, vary consciously and sparingly. This reverses current practice, where every development invents its own geometry. Instead of proving why you should follow time-tested patterns, you must justify why you should deviate. The burden of proof shifts to the innovation, not the tradition. ## The 90% Solution These four rules capture perhaps 90% of what is needed for a successful foundation of urban form. Master them and you've created a framework for places that can thrive for centuries. The remaining 10%—the art of street networks, the placement of parks, the response to topography—matters too. But without getting these fundamentals right, no amount of artistry can save a place. The next chapter explores that remaining 10%—how creativity and variety work within the framework these rules establish. But first, internalize these four principles. They're simple enough to remember, flexible enough to adapt, and powerful enough to transform your neighborhood, town, or city now and for the next thousand years. --- Continue to [[Chapter 7 - Supplementary Rules]] →