The 'grid system' that Chicago's streets were laid out with has a 'primary' street at each half mile, the equivalent of four city blocks; and eight city blocks measure one mile (however, there is a discrepancy to this in downtown where some streets do not always measure a full block, or 1/8 mile). The origin point is in the heart of the loop at State St (0W/E) and Madison Ave (0N/S). State St runs North/South along the 0W/E coordinate. All other streets that run North/South are along a coordinate that represents how far East or West it is from State St and the 0W/E coordinate. The first street East of State St is Wabash Ave, along the 45E coordinate, next there is Garland Ct at 70E, then Michigan Ave at 100E, and so on as you continue heading East. So, the coordinates for the intersection of Michigan Ave and Madison Ave would be 100E & 0N/S. Furthermore, each city block is marked in increments of 100, which tells you that Michigan Ave is one block East of State St.

For example, Halsted St runs North/South and is 8 blocks West of State St at 800W. Since there are 8 blocks to a mile, we can also determine that Halsted St is 1 mile West of State St. The street that is 24 blocks North of Madison Ave is Fullerton Ave, referred to as 2400N, and is three miles North of Madison Ave. This simple system makes it very easy to surmise where you are in relation to downtown; knowing a cross street's coordinates, it's effortless to find any address in the city. An address of 2400 N Halsted St will be three miles, or 24 blocks, North of Madison Ave. Knowing that Halsted St is 800W, one may also assume the address is one mile, or 8 blocks, West of State St. Since addresses are numbered evenly on the North or West side of a street, while odd numbered addresses will be on the South or East side of the street, we can also tell that this address is on the North-West corner of the Halsted St/Fullerton Ave intersection. A building with an address of 799 W Fullerton Ave will be on the South-East corner, across the intersection.