Piqua is the city that almost hosts all of the adventures of the Captain Underpants series, and is situated in Ohio.
Piqua was also the place where the two main characters, George Beard and Harold Hutchins live. They meet up in a treehouse near their homes where they create their most famous comic books, such as "Timmy The Talking Toilet", "The Amazing Cow Lady" and the most well known series "The Amazing Captain Underpants". Their principal, Mr. Benjamin Krupp, also lives at Piqua. He lives on Curmudgeon Boulevard, as described in "Captain Underpants and the Preposterous Plight of the Purple Potty People". It is also described that his house is covered with toilet paper and he doesn't even know it.
There was also another version of Piqua in Captain Underpants and the Preposterous Plight of the Purple Potty People. George and Harold travelled to the "Purple Pottyville", which there were many differences between the real one. For example, the bad guys in the real Piqua, Dr. Diaper, Turbo Toilet 2000, The Naughty Cafeteria Ladies, Professor Poopypants, and the Robo-Boogers, has been automatically transformed into good guys.
Chapter 2 of Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies from Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds) begins "One dark, clear night in Piqua, Ohio, a flaming object was seen streaking across the quiet midnight sky. It shone brightly for a second or two, then fizzled out just above Jerome Horwitz Elementary School. Nobody gave it a second thought."
In Captain Underpants and the Perilous Plot of Professor Poopypants, George and Harold wrote a comic book about Professor Poopypants which is known as "Captain Underpants And The Pied Pooper Of Piqua" , and in the end of the story,Professor Poopypants has been taken to the "Piqua State Pennitery", which is also written as "PIQUA STATE PENN" on Professor Poopypants' letter to George and Harold.
According to Wikipedia, Dav Pilkey chose Piqua as the setting of the Captain Underpants series because an underwear festival was held there once a year.