Steve Rushin of Sports Illustrated has called the line running through Connecticut that separates Yankee fans and Red Sox fans the Munson-Nixon line. Mr. Rushin came up with the name — in honor of the late Yankee catcher Thurman Munson and the retired Red Sox right fielder Trot Nixon — in 2003, and he had to guess where the line ran: “north of New Haven but south of Hartford, running the breadth of central Connecticut.”

We don’t have to guess anymore.

Fans may not list which team they favor on the census, but millions of them do make their preferences public on Facebook. Using aggregated data provided by the company, we were able to create an unprecedented look at the geography of baseball fandom, going down not only to the county level, as Facebook did in a nationwide map it released a few weeks ago, but also to ZIP codes. We can now clearly see that both Hartford and New Haven are in fact Yankee outposts. We can also determine the precise Chicago neighborhoods where White Sox jerseys stop being welcome and the central California town where the Dodgers cede fan favorite status to the Giants.

We’ve created two features to help readers explore the data. First is an interactive map of the United States that allows you to explore not just the most popular team in your neighborhood but also a table of the top teams for any ZIP code in the country.

Second, in the spirit of Mr. Rushin’s Munson-Nixon line, we've generated 14 maps detailing baseball’s biggest rivalries, highlighting the borders and offering suggested names for those lines.

The maps were created using estimates of team support based on how many Facebook users “liked” each team in a ZIP code. We applied an algorithm to smooth the data and fill in gaps where data was missing.

If you have alternate suggestions for border names, or have an idea or suggestion for a map not shown here, leave us a message in the comments or on Twitter or our Facebook page. We'll be updating this list regularly. Hopefully, this is just the start.