I wonder whether "staircase" and "stair" have different meaning. It feels like the same to me.
A "stair" is any step that is part of a series leading to a different floor or level. When you have multiple stairs that is when the plural form is used. A staircase is a structure that holds a series of stairs. It is one solid structure. So while all staircases have stairs not all stairs are staircases. Ex: There are 3 stairs so I need help with my wheelchair. Ex: A. That is a beautiful staircase! B. Yes, it leads to the rooftop.
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