What are the minimum standards of tznius for halachic purposes? By "halachic purposes" I mean Shulchan Aruch, Orach Chaim 75, which forbids praying in the presence of ervah. I am raising the question in light of the discussion begun here, which deals with the fact that different communities can have vastly different standards of tznius.
Which body parts must absolutely be covered from a halachic perspective? If a woman shows, for example, her collarbones, elbows, or hair because her minhag permits it, is she invalidating (or worse) the prayers of those around her with a different minhag? And, tangentially, what would those others be obliged to do in such a situation? I am thinking of kiruv settings, among others.