Chicago streets 1: The mystery of the South Side

Everyone knows that Chicago streets are numbered and every eight blocks is a mile. (Incidentally, the system was introduced only in 1908, before which there was street numbering chaos.) On the North Side you just have to memorize the numbering - Division is 1200, Lawrence is 4800, etc. But on the South Side, all east-west streets are named after their number, with a couple exceptions like Roosevelt and Garfield. You would expect the main streets to fall on the fours since there's a main street every half-mile. But as anyone who's taken the Red or Green Lines knows, the main streets instead land on places like 47th, 63rd, 87th, 111th.

The problem is not that the South Side has forsaken the principle of a main street every half-mile. It's that South Side streets near the Loop were already numbered when the street numbering reform went thru in 1908, and they didn't match up exactly with the new system. Instead of renumbering those streets, they were left as is, while the newer parts of the South Side were integrated into the 800-to-a-mile system.

Roosevelt (1200 S) is actually one mile south of Madison (1 N/S), Cermak (2200 S) is two miles south, and 31st (3100 S) is three miles south. After that the regular system resumes, which is why the main streets then follow regularly: 39th, 43rd, 47th, 51st, 55th, etc.