Projectionist

Chicago was built on a grid system with a “major” street every half mile – equal to four blocks. The starting point is at the intersection of State and Madison in the heart of the Loop. State Street runs north/south. All of the other north/south streets are numbered as so many blocks east or west of State, with the addresses increasing by 100 for each block (except downtown, where streets do not always correspond to a full block).
For example, Ashland Avenue runs north/south and is 16 blocks west of State and so its “address” is 1600 West. Since there are 8 blocks to a mile, we can also determine that Ashland is 2 miles west of State. Similarly all east/west streets are numbered as north or south of Madison, which runs east/west. The street that runs parallel to Madison eight blocks to the north (Chicago Avenue) is referred to as 800 North, and is one mile north of Madison.
Using this system you can always figure out how far and in what direction you have strayed from downtown, and by knowing a cross street’s address you can find any building in the city. A building with an address of 800 North Ashland Ave. will be 8 blocks north of Madison on Ashland which is 16 blocks west of State Street, at the intersection of Chicago Ave. and Ashland Ave. A building with an address of 1601 West Chicago Ave. will be on another corner of the same intersection.
(from http://www.transitchicago.com)