My understanding is that “chão” is used to refer to the ground outside or the ground floor in a building.
Is there a difference between “andar” and “piso” when referencing the floor of a building?
@jrlitton, chão not only refers to the ground outside, but also to any floor you’re walking over (e.g. kitchen floor = chão da cozinha; wood floor = chão de madeira). The ground floor of a building is specifically called rés-do-chão (and we don’t abbreviate this). As for other floors of a building, you can use andar and piso interchangeably - there’s no practical difference between them 
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Muito obrigado!
This had been confusing me for some time
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