There isn't just one because we are a very mixed nation that was born out of multiculturalism. So, it would mean that there would have to be a series of American ethnic costumes like Native American dress, Cowboy, Colonial attire, Afro-Creole Clothing, Slave Rags, Immigrant Clothing (Asian and European), Hawaiian Traditional Dress, and Hispanic Southwestern Style. So you see there really isn't just one national costume for the USA or for many other countries in the Americas. Like Mexico has several costumes as does Brazil. In countries like USA, Canada, Brazil, Mexico, Cuba, and Australia it is hard to define a pure national costume. There really is no such costume here in the USA. The culture and demographics change depending on the section of the country you are in.
Probably blue jeans and a white t-shirt for both! Oh and an American flag somewhere like a bandana or pin.
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