Apology to Safari users:
To enter the system, you begin by choosing a location to begin mapping. Select whether you want to begin with a particular city (of the 39 that will eventually be mapped), or with a state or county. If you choose a city, the map will open to that city and you will be able to choose which data to see at the Enumeration District level (these are variables listed in the menu as EDH, for "Enumeration District – Historical"). If you choose a county or state, the map will open with a choice of the same variables at the county-level (listed as CTY, for County).
You can then use the navigation tools (along the top) to show a larger or smaller area and to move from side to side. You can return at any time to the original selection page ("search and map"). Then choose a specific variable to map from the menu on the left-hand side of the page. As you zoom out from a city, the city map layer and ED variables disappear, and you see only the county boundaries. At this scale, the menu only includes county-level variables. But if you zoom back in, the ED detail will reappear.
This is a versatile system. A key feature (listed on the left-hand side) available when you are viewing EDs within a city is the "historical image." This is a historical street map that will be visible as a layer underneath any other variable that you choose to map, but it can be turned off if you prefer just to see the ED map. The "identify" tool (along the upper edge) lets you see the exact data for an ED or county by clicking on it. The "select by rectangle" tool allows you to download data about a specific set of EDs or counties. The "query" tool allows you to find areas that meet specified criteria. With the "print" tool you can create a finished map with a title and legend in a separate window, and you can then use browser editing tools to copy it to any application. A help window (click on Help in the upper right corner) is available to explain each tool separately.