Which parts of a womans body must be covered according to basic halacha? What should be covered for tznious purposes?
There are certain parts of the body that must be covered irrespective of the local custom. These are the thighs (down to the knee), the upper body, the arms (as ruled by the Mishnah Berurah) up to the elbow, and a married woman’s hair.
Beyond this, the halachah depends on the local custom of women.
The halachos pertaining to these matters are found principally with regard to reciting Kerias Shema in front of a woman who is not modestly dressed. However, the same principles apply to the tznius required of women in the puclic domain.