Hello kmeyers. Here is a clip of the Chicago Sectional Chart. Each tick mark of latitude equals one nautical mile, which is 1.15 statute miles. You can see "Chicago Class B" printed over the outer blue circle over Lake Michigan. There is a "donut ring" denoted by blue circles for an area encircling O'Hare that also runs between DuPage airport and Schaumburg airport. This is about 15 miles from O'Hare, so you are probably somewhere within this ring area. You will see a set of numbers "100/30" printed within this ring. This is the top and bottom of Class B shown in hundreds of feet above mean sea level (MSL). This means the base of Class B airspace starts at 3000 feet MSL and the top of Class B air space is at 10,000 feet MSL. Since DuPage has an elevation of 759 feet above sea level and Schaumburg is at 801 feet MSL, the average ground elevation in this area of the ring would be around 780 feet which would mean the base of Class B would start at about 2220 feet above the ground in this area. The dashed blue circle around DuPage depicts Class D airspace which starts at the ground and tops at 3300 feet MSL. The ring that includes Schaumburg airport shows 100/19 which means the Class B base starts at 1900 feet MSL and tops out at 10,000 feet MSL. There is a magenta shaded line running up Lake Michigan, across the top of the picture and down the left side of the picture and under the Chicago area outside the picture (essentially encircling the entire Chicago area). Everything inside this magenta shaded outline has Class E airspace starting at 700 feet above the surface instead of 1200 feet.. This means there is no Class G airspace between 700 and 1200 feet in this area. Class E airspace starts at 700 feet above the surface and goes up to the base of Class B airspace, except for some areas of Class C around Midway and Class D airspace around DuPage and Chicago Executive airports and the class B airspace immediately around O'Hare which starts at the surface for the ring around O'Hare (depicted by 100/SFC), within which there is no Class G or Class E airspace.