Must a married [or once married] woman cover every strand of hair of her head?
It is forbidden for a married woman to reveal even a single hair of her head to the public, and hence all of the hair, must be covered. It is for this reason that many of today’s Poskim rule one is to wear a wig [Sheitel] rather than a head Kerchief [Tichel], as the wig guarantees the covering of all the hair, as opposed to a head kerchief. However, some Poskim of today novelize that according to Halacha, a woman is permitted to be lenient to uncover up to two finger-worth’s [4 cm] of the top part of her hair The Rebbe, as well as the vast majority of today’s Poskim, negate this opinion, and state it has no place in Shulchan Aruch. Those women who uncover even more than 4 cm of their hair are to be protested according to all. Furthermore, everyone agrees that according to the Zohar it is a complete prohibition to reveal any hair of the head, and one who is not careful in this causes poverty and spiritual challenges for their children, as well as other Tzaros.
 Tzemach Tzedek Even Haezer 139/2 from the fact he prohibits the sideburns; Chasam Sofer 36 “Any hair in any area of the head is an Erva”; Ashel Avraham Butchach “Proper to beware against even one hair”; Sdei Chemed; Dovev Meisharim 1/124; Teshuvos Vehanhagos 1/62; Beis Baruch on Chayeh Adam 4/10; Lehoros Nasan 5; Piskeiy Teshuvos 75/10; Chazon Ish in Dinim Vehanhagos 2/88-9 and in Teshuvos Vehanhagos 1/62; Rebbe Ben Tziyon Aba Shaul brought in Kuntrus Kisui Rosh Leisha; Rav Mashash in Tevuos Shemesh 137; Shemesh Umagen 2/15-17; Az Nidbaru 12/41 that one should cover every hair; Rav Moshe Shternbuch in Daas Vehalacha 1; Rav Lefkovich in Kovetz Beis Hillel 9; Or Yitzchak 3; Nitei Gavriel in Kovetz Or Yisrael 36-37
 Poskim ibid including: Chazon Ish ibid; Rebbe Ben Tziyon Aba Shaul ibid; Or Yitzchak 3 in name of Rav Moshe Feinstein; Letters of Rebbe printed in Shulchan Menachem 6/80-84
 Igros Moshe 4/112; Even Haezer 1/58; Likewise see Or Yitzchak 3 in name of Rav Moshe Feinstein that even he agreed that initially one should wear a wig in order to cover every hair; Rav Ovadia Yosef in Yalkut Yosef Otzer Dinim allows Bedieved
The reason: As only a Tefach is considered an Erva by a woman [Igros Moshe ibid]; and less than a Tefach is only Rabbinical, and hence less than majority of a Tefach is allowed. [Rav Ovadia ibid]
 Igros Kodesh 9/325 [printed in Shulchan Menachem 6/81] “You want to wear a Sheital that will leave two finger-worth’s uncovered! Who are you trying to fool, not the world, but only yourself, and what is the purpose of fooling yourself? The rule is clear in Shulchan Aruch [that all the hair must be covered] and there is no room for question on this matter.” See also Igros Kodesh 16/330 and other letters printed in Shulchan Menachem 6/80-84
 All Poskim brought in previous footnotes
 Parshas Nasso
 Brought in Maharam Alshiker 35; Chasam Sofer ibid