The Tri-State Area is the name of the metropolitan area surrounding Danville. It was founded by John P. Tristate, who united the Bi-State Area with an adjacent area, founded by Otto H. Adjacent. Tri-State Area Unification Day is celebrated on July 15 ("Hip Hip Parade").
Roger Doofenshmirtz is the current mayor of Danville. His brother, Heinz Doofenshmirtz often plans to create chaos in this area, not just the city of Danville itself. For such purposes, he once built a scale model of the Tri-State Area.
Some businesses and groups have "Tri-State Area" in their name (e.g. Tri-State Area Public Library, Tri-State Bombers, etc.).
While describing one of his schemes, Doofenshmirtz used a magnifying glass in front of a map of the United States to reveal the Tri-State Area's location. The magnifying glass was positioned near the Denver, Colorado area, which is within driving distance of Mount Rushmore ("Flop Starz", "Candace Loses Her Head").
One scale model by Heinz Doofehnshmirtz depicts a geographical area of less than one square mile. However, another larger scale model was made by Doofenshmirtz; the scale of the model is unknown ("Toy to the World", "That Sinking Feeling").
It is presumed, while the Tri-State Area is scattered throughout episodes to fit into the story, that Danville is in South Dakota or at least a half hour away, as numerous episodes support this theory. On the trip to Mount Rushmore ("Candace Loses Her Head"), Candace's excitement over going to the mall tells us it was close enough to be kept a surprise. If Mount Rushmore were further away, further planning and a longer road trip would be required, and she would know. In both rollercoaster episodes ("Rollercoaster", "Rollercoaster: The Musical!"), the Rollercoaster cars are flung to Mount Rushmore, this may be a sign that it was close (it could have been farther, though, as distance apparently wasn't an issue, as they were flung to Paris, France). In "Just Desserts", Linda's map shows the location of the Tri-State Area somewhere in the Central-Northern United States, almost over South Dakota.
- Series creator Dan Povenmire has stated that Danville is not in any specific geographical location. The surrounding region changes to fit the needs of the episode's story, leading to Danville and the Tri-State Area being near both Mount Rushmore and the Atlantic ocean at the same time.
- In the real world, a tri-state area need not encompass whole states. For example, Chicagoland includes several counties in northeast Illinois along with a few in the most northwest part Indiana and one in the most southeast part of Wisconsin.
- The close proximity of the Atlantic Ocean suggests that Danville is in the New York Tri-State Area. Additionally the adjacent and bi-state-areas are alligned just like New York, New Jersey, and Connecticut. However, the east part of the bi state area doesn't include Long Island, and the town's attitude towards the statue unvealed in Candace Against the Universe doesn't quite match up with New Yorkers' attutudes.
- There's a town called Danville south of Lexington, Kentucky, which is in the general area of the Ohio-Kentucky-West Virginia tri state area. Additionally, a map of the Tri-State Area shown under Doof's hourglass shows a very similar boarder, although it seems to be further west.
- There is also a Quad-State Area where the Regurgitator's base is located. It's mentioned while Major Monogram is briefing Agent P during the beginning of "Oh, There You Are, Perry". Other than the fact that the Regurgitator had built his base here, not much is known about the Quad-State Area. Most likely, it is located on the border of Utah, Colorado, New Mexico, and Arizona. So far it has only even been mentioned in the episode "Oh, There You Are, Perry". Judging by the fact that Doofenshmirtz drove there in such a short time, it's most likely close to the Tri-State Area.
- The logo/flag for the Tri-State Area was first seen in Phineas and Ferb the Movie: Across the 2nd Dimension.
- Prior to Phineas and Ferb the Movie: Across the 2nd Dimension, it's 2nd Dimension counterpart was ruled by the 2nd Dimension Heinz Doofenshmirtz.
- It has many counterparts, one in "Tri-Stone Area" named "Tri-Stone Area", another in "Doof Dynasty" named "Tri-Province Area", and one in "Excaliferb" named "Tri-Kingdom Area".