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.

New York Yankees vs. Boston Red Sox

Munson-Nixon Redux

We begin with a new take on the Munson-Nixon line, which differs from some previous interpretations in that it shows that Hartford has declared for the Yankees. It’s still a border town, but it's a border town the way El Paso is — clearly on the Yankee side. One tidbit: Red Sox Nation seems to have conquered some new territory since 2012, when Ben Blatt tried to draw such a line. (Winning a World Series will do that.) We also encourage you to read our colleague John Branch's attempt at drawing the line in 2006, which is quite impressive and accurate given his shoe-leather means.

New York Mets vs. New York Yankees

The We’d-Draw-a-Line-If-We-Could Line

There’s no other way to put it: The Yankees are the preferred team everywhere in New York City, and nearly everywhere in the U.S. over the Mets (in more than 98 percent of ZIP codes nationwide). One small Mets bright spot, if you want to call it that, is that in the area in Queens surrounding Citi Field, where the Mets play, Facebook users had the courtesy to prefer what some call the Evil Empire at a slightly reduced rate relative to its advantage elsewhere in New York.

Minnesota Twins vs. Milwaukee Brewers

The Molitor Line

Minnesotans and Wisconsinites don’t admit to having much in common, but in sports they have sometimes been forced to share. The most notable example is probably Paul Molitor, for whom our border is named. Molitor, a Hall of Famer from St. Paul whose name sounds conveniently similar to Maginot, played most of his career with the Brewers but finished it with the Twins. The teams’ fans display a certain Midwestern stubbornness in keeping their allegiances roughly to state lines, with the exception of those WINOs (Wisconsinites in name only) who live just over the river from the Twin Cities. There’s no real animosity between these two teams, given that they rarely play each other now. Still, don’t put the peace at risk by bringing up Brett Favre.

Chicago Cubs vs. Chicago White Sox

The Red Line Red Line

Helped by the charm of their field and by WGN’s national broadcast reach, the Cubs have always been the better-loved of Chicago’s two teams. But the White Sox — unlike the Mets or the A’s — do have a patch of their own territory, on the city’s South Side and into the southern suburbs. Interstate 290 (the Dwight D. Eisenhower Expressway) is a rough guide to the border: Know whether you're north or south of it before you put on your hat.

Los Angeles Dodgers vs. Los Angeles Angels

The Reagan-Nixon Line

Among all the second-favorite teams — including the Mets, the A’s and the White Sox — the Angels have done perhaps the best job of carving out a niche of fans. But despite the inroads made by the Angels, the Dodgers are still the clear favorite in Southern California. Their territory extends east to Big Bear Lake and west to Seal Beach. Also perhaps in the Dodgers’ favor: The Ronald Reagan Library is in their territory, while the Angels have the Richard Nixon Library.

San Francisco Giants vs. Oakland Athletics

Giant Country

Like the Mets, the Athletics are the less popular team in a two-team region — less popular everywhere in that region, based on the data from Facebook. Again, winning the World Series matters. The Giants have won two of the last four. The A’s have won none of the last 24.

Oh, Ohio

On to Columbus

Few states give their fans as many options as Ohio does. A fair-weather fan in the central part of the state could claim allegiance to several teams within a day’s drive: not just the Reds and the Indians but the Cubs, the White Sox, the Pirates, the Tigers or even the Cardinals. But most Ohioans stay loyal to the home-state teams, and the border is right where you'd guess. It runs straight though Columbus, Ohio’s capital and largest city, in the middle of the state. One small exception to the Reds’ and Indians’ dominance: The longtime presence of the Tigers’ top minor league team in Toledo — the Mud Hens, the favorite team of Corporal Klinger from the television show "M*A*S*H" — has helped the Tigers claim some territory in northern Ohio.

St. Louis Cardinals vs. Kansas City Royals

The Denkinger Line

The Show Me State has mostly thrown in with the Cardinals, who have been as good for the last two decades as the Royals have been bad. The line is well west of the halfway point between the two cities — Columbia, Mo. — and instead runs through Sedalia and Pittsburg. We can only guess what this map would have looked like in 1985, when a missed call by Umpire Don Denkinger helped the Royals win their last World Series, over the Cardinals. Unfortunately, there’s no Facebook map for that: Mark Zuckerberg was not yet 2 years old.

New York Mets vs. Philadelphia Phillies

The Sub-Hoagie Line

A common mistake in data analysis is to confuse correlation with causation; just because things are related doesn’t mean one caused the other. At the same time, last year's New York Times analysis of American dialects allows us to notice a striking pattern: Which N.L. East team you root for may be revealed by whether you call long sandwiches subs or hoagies. We don't know which causes which, but we do know they go together.* (Because of a mapping error, an earlier version of this map incorrectly characterized Trenton residents as Mets fans who eat hoagies. They appear to be Phillies fans or Yankees fans who eat hoagies.)

Houston Astros vs. Texas Rangers

The Nolan Ryan Line

The state of Texas belongs to the Rangers, and why not? The team claims to represent the whole state, whereas the Astros belong merely to Houston, at least going by their name. It doesn't hurt that the Rangers have made the postseason three times in four years, while the Astros, who have been dreadful for several years, can't decide on a logo or even seemingly what league they're in. Many Houstonians also take pride in being a bit different from other Texans. Those other Texans apparently agree.

Washington Nationals vs. Baltimore Orioles

The Line of Potomac Aggression

After the departure of the Senators in 1971, Washingtonians spent decades alternately lamenting their plight and treating the Orioles as their home team. That era is over. While the Orioles have held onto many Maryland suburbs, the Nats now dominate Washington and much of the Virginia suburbs. In parts of the Washington region, the Orioles aren’t even the second-favorite team; the Yankees or the Red Sox are.

Chicago Cubs vs. St. Louis Cardinals

The Downstate Line

Most states with a team have few areas that side with an out-of-state rival. Not so in Illinois, where half the state stands in open revolt. Bloomington, Champaign and Peoria appear to belong to the Cubs, but south of those cities, expect to see a lot of Cardinal red. If you’re a longtime resident of this area, you may want to look up whether your ancestors voted for or against Abraham Lincoln in 1860.

Baltimore Orioles vs. Philadelphia Phillies

The Morgan-Ripken Line

The South may have lost the Civil War, but the Orioles have done a better job of pushing north above the Mason-Dixon line than the Phillies have done expanding beyond their home state. Notably, the Orioles have won over Gettysburg, Pa. We’ve named the Orioles-Phillies border for two of the protagonists in the 1983 World Series, which the Orioles won in five games.

Southern California

Highway 1 Passport

Southern California is a group of small nation-states. San Luis Obispo, often considered a halfway point between the Giants and the Dodgers, is squarely Dodgers territory, according to Facebook users. Further south, the Padres have problems. The baseball writer Joe Sheehan has noted their major challenges in raising revenue, given that they are hemmed in by the Pacific Ocean, the Mexican border, a desert and the Dodgers and the Angels. No one symbolizes the plight of the Padres, who have never won a World Series, quite as well as Adrian Gonzalez. Born in San Diego, he spent five years as a Padres slugger. But they couldn’t afford him, and he now leads the Dodgers in home runs.

*Note: The New York Yankees were removed to make this map possible.