Brachos 24 tells us that the exposed “shok” of a woman is considered ervah in the presence of which kri'as shema cannot be read (see Shulchan Aruch O.C. 75). What is the part of the body known as the “shok”? The Pri Megadim, based on the Rambam’s description of the parts of an animal (Ma’aseh Korbanos 9:10), writes that the “shok” is from the knee and above. The Mishna Berura cites the PM”G and notes that as long as the common practice is not to cover the body below the knee it is no different than the hands, face, etc. (Just a note to avoid confusion: the "shok" is an ervah regardless of whether people in society expose that area or not. Outside that area is dependent on social norma).
The discussion does not quite end there. Even though in the context of ervah it may seem reasonable that the term “shok” refers to the upper leg, the Shulchan Aruch elsewhere seems to use the term to refer to the lower leg. The S.A. writes with respect to chalitzah that the shoe is tied on the “shok” (E.H. 169:28). The proof of the PM”G from animal parts is also not so clear cut, as Tosfos (Menachos 37) writes that the terminology that is applied to animal parts is not necessarily analogous to corresponding parts of the human body. The Chazon Ish raises these issues as a matter of discussion, but does not go far as to say the Mishna Berurah is wrong. Others, such as the R' Shmuel haLevi Wosner (in the first tshuvah in his sefer), are adamant that the entire lower leg must be covered.
Nafka minah l’dina and l'shopping: are your daughter’s skirts supposed to drag to the floor, or just cover the knee?