In 1620, an Englishman by the name of Edmund Gunter invented a land surveying device known as Gunter's chain. As the name suggests, it was an actual chain (see above). Each chain contained 100 links and, when fully extended, it measured 66 feet.

This was a monumental innovation as it greatly simplified land surveying and made it a lot easier to measure out acres -- especially if you maybe weren't great with math. So it is perhaps no surprise that this simple device forever changed our cities.

But first, here's the only math you need to know:

Number of chains x number of chains / 10 = number of acres

For example:

A lot measuring 66 feet by 66 feet would mean it has an area of 4,356 square feet, or 0.1 acres (1 acre = 43,560 square feet). It would also mean that this lot measures 1 chain by 1 chain, or 1 square chain. Take 1 square chain and divide it by 10, and you arrive at the same 0.1 acres.

Similarly, a lot measuring 660 feet by 660 feet would mean it has an area of 435,600 square feet, or 10 acres. Using Gunter's chain, this lot is 10 chains by 10 chains, which equals 100 square chains. Divide 100 square chains by 10 and you arrive at the same 10 acres.

Put differently, 1 acre equals 10 square chains in Gunter's system.

Because of its simplicity and utility, the chain became a statutory unit of measurement in England by the 1670s. And as a result, it spread throughout the British Empire, meaning it started to influence how new cities were being planned and laid out.

Let's look at the example of Salt Lake City.

We have spoken before about the city's famously large blocks. They have the dubious distinction of being the largest in the US. But what you may not have noticed is that the typical SLC block measures exactly 660 feet x 660 feet. Its typical streets are also 132 feet wide.

This is because of Gunter's chain. These are 10-chain x 10-chain blocks and 2-chain streets.

The same is true of other cities. Looking on the other end of the spectrum, Portland's compact street grid is comprised of blocks that measure 198 feet x 198 feet. These are, in other words, 3-chain blocks. Its typical streets are also 33 feet wide. So half-chain streets.

Units of measurement have a lasting way of influencing how we plan and design things. This is true at small scales and it's also true of our cities. In tomorrow's post, we'll look more closely at Salt Lake City's street grid and what it does to walkability.

Image: National Museum of American History