Flags appear in every main game in the series (excluding the original Doubutsu no Mori), and can be altered to display a custom design. They primarily serve as an emblem representing the town, though other kinds of flags appear as well.
In the games
A flag with a turtle-shell design is flown next to the Town Gate. The design can be changed by speaking to Booker in the Town Gate and selecting the option to 'Change flag.' The player has the option of outright replacing the design with a new one, or swapping a custom design in exchange for the one used for the flag.
A flag with a tree design appears outside of the Town Gate and is also displayed on top of the New Year's Eve countdown board. As with Wild World, the design can be changed by speaking to Booker in the Town Gate and selecting the option to 'Change flag.' The player has the option of outright replacing the design with a new one, or swapping a custom design in exchange for the one used for the flag.
After the player fully pays off their mortgage Tom Nook will gift them a personal flag outside of their house, which initially bears Tom Nook's leaf sigil in a color matching their roof paint. The flag can be changed by selecting the option from the telephone in the attic of their house.
The town's flag is flown outside of the Town Hall and on the platform of the Train Station. The design can be changed by speaking to Isabelle in the Town Hall and selecting the 'Town flag' option. Kapp'n's boat flies the flag of Tortimer Island, the design of which cannot be altered.
A special flag design in the image of a pine tree can be obtained from the official Japanese Nintendo Dream Town. Additionally, two of the default designs are an alternative rose flag and a stag-beetle flag.
The default town flag, Tortimer's Pride, in Wild World
The default Town Flag in City Folk
The distributed Mayor's Flag design in City Folk
The default Town Flag in New Leaf
The Rose Flag default design in New Leaf
The Stag-Beetle Flag default design in New Leaf
A player's personal flag being flown outside of their house in City Folk