Must a married [or once married] woman cover her sideburns and others hair that protrude past the regular hair of the head?
Some Poskim rule that a married woman is not required to cover the little amount of her hair that extends past her “Tzemasan” [which refers to a snood or other regular hair covering]. [This allowance includes her sideburns and any other hair that protrudes past the head, such as by the back of the neck and below the ear. Some write the rule is that all hair that does not grow long like regular hair is considered as the “Tzamsan” which is not required to be covered. Some Poskim suggest that perhaps even according to the Zohar, who is generally very stringent against a woman revealing even one hair, it is permitted for these areas of hair to be revealed.] Other Poskim however rule it is forbidden for any of the above hairs to be revealed, and it is thus required to be covered just like any other hair of the head. Furthermore, some Poskim rule that even according to the lenient opinion, according to the Zohar who prohibits to reveal any hair of the head, this hair is also included, and one who is not careful in this causes poverty and spiritual challenges for their children, as well as other Tzaros. Practically, one is to be stringent in this matter.
Areas where the custom is to be stringent: In all areas or communities in which the custom is to be stringent to cover even hair that extends past the head, such as the sideburns and hair below the neck, it is forbidden according to all opinions for a woman in such a community to reveal these areas of her head. This applies even if the woman is affiliated with another community or city which is accustomed to be lenient in this matter, nevertheless, when she is in the area of the stringent community, she must be stringent. In the case that a woman moved from a community which was stringent in this matter to a community that is lenient, then according to the lenient opinion mentioned above, she may likewise be lenient of she intends to live in that new area. According to the stringent opinion, it is forbidden in all cases.
In the privacy of her home, in front of her husband: Even according to the stringent opinion mentioned above, it is permitted for a wife to uncover the sideburn areas and neck areas of her hair when privately with her husband. This applies even at times that she is a Nida. [However, some Poskim rule that according to the Zohar, even this is forbidden, even in the presence of her husband, and even in times that she is not a Nida.]
It is disputed amongst Poskim as to whether a woman must cover the hair that extends past her head, such as the sideburns and hair that borders her neckline by her back. Practically, according to the implication of the Zohar, it is forbidden to reveal even these hairs, and every woman should be stringent in this matter. According to all Poskim, it is forbidden from the letter of the law to reveal these hairs in an area or community which is accustomed to be stringent in this matter. From the letter of the law, it is permitted for a wife to reveal these hairs in privacy in the presence of her husband, even if she is a Nida, although according to the Zohar, it is possibly forbidden to do so.
Does neck hair need to be covered?
Hair that borders the neckline of the back of the neck, and usually protrudes when wearing a snood, is relevant to the above dispute regarding whether it must be covered, of which the Chabad ruling is to be stringent. However, those hairs which clearly grow on the neck, and are not part of the head at all, do not need to be covered, although some are accustomed to shave it off, seemingly due to Maras Ayin.
How much of the sideburns must be covered?
All hair that grows opposite the ear must be covered according to the stringent opinion. All hair that grows below the ear, is not part of the sideburns and is not required to be covered.
 Admur 75/4 “So too, the hairs of women which regularly protrude past their Tzamatan in some lands, is permitted to learn Torah and Daven in its presence, as since the people are accustomed to see this area of hair revealed, it therefore does not lead to erotic thoughts.”; Rama 75/2; Rashba Brachos 24a in name of Raavad; Maharam Alshiker 35 based on Aruch and others that such hair is not required to be covered; Biur Halacha 75/2 “Michutz” “This allowance applies to all women, unlike the Chasam Sofer who was stringent in this”
 A Tzematan is a knitted snood which is placed over the hair to gather it together in one bunch. The little bit of hair that cannot be gathered into the snood, and hence protrudes to the outside is the hair referred to in this Halacha. [Chasam Sofer 36; M”B 75/14]
 Maharam Alshiker ibid “The hair between her ear and forehead”; See Chasam Sofer 36
 Shevet Halevi 5/207 that certainly from the letter of the law there is no need to cover the hair which grows on the actual neck, and those who choose to shave it off, seemingly do so due to Maaras Ayin.
 Piskeiy Teshuvos 75/10
 Maharam Alshiker ibid although he concludes that even if the Zohar prohibits even these hairs, we nevertheless rule like the Tlamud against the Zohar
 Tzemach Tzedek Even Haezer 139/2 and Mishnayos Brachos 3/5-3 [The Tzemach Tzedek rules that the ruling of the above Poskim [Rashba, Rama and Admur ibid] was only said regarding a wife and husband and not regarding other people.]; Implication of Beis Yosef 75 in name of Rashba ibid that the allowance only applies to the husband; Chasam Sofer 36 that although according to the Talmud it is permitted, we rule like the Zohar, as is seen from the custom of Jewry to be stringent; Implication of Orchos Chaim Kerias Shema 36; Shevet Halevi 5/15 “One is to be stringent like their words.”
 Chasam Sofer ibid; M”B 75/14; Kaf Hachaim 75/18; Possible way of learning M”A ibid; See Maharam Alshiker ibid who so implies in his conclusion
 Parshas Nasso
 Brought in Maharam Alshiker 35; Chasam Sofer ibid
 Shevet Halevi ibid;
The Chabad ruling: Seemingly, the Chabad ruling should follow the stringent opinion, as we follow the rulings of the Tzemach Tzedek, at times even when it contradicts the ruling of Admur, and certainly in a case where the Tzemach Tzedek is more stringent. Vetzaruch Iyun. Nonetheless, in any event based on the Zohar one is to certainly be stringent.
 M”A ibid in name of Mahram Alshiker ibid; See Admur ibid who writes “Who are accustomed to do so in some lands,” thus implying like the M”A that this matter is dependent on Minhag, and is not a clear cut allowance to all areas. See Chasam Sofer ibid
 The reason: Seemingly, this is forbidden due to two reasons as a) If the custom is to be stringent to cover these areas, then uncovering it would transgress Daas Yehudis, and cause erotic thoughts to the men of that community who are not accustomed to seeing this hair uncovered. [See Admur ibid and Chasam Sofer ibid] b) It causes dispute when one is lenient in a community that is accustomed to be stringent. [See Admur 468/10]
 See Admur chapter 468 that she must be stringent like the stringencies of the new town even if visiting.
 M”A ibid in name of Mahram Alshiker ibid; See Admur 468/12-14, Vetzaruch Iyun regarding Admur’s ruling to avoid doing the stringency in public due to Machlokes, and how that would apply in this case.
 Beis Yosef 75 in name of Rashba; Tzemach Tzedek ibid; Chasam Sofer ibid; Shalmas Chaim 64
 Chasam Sofer ibid; Possible way of learning all Poskim ibid who rule or imply that according to the Zohar even these hairs may not be uncovered, and hence this would apply even to the husband.
 Shevet Halevi 5/207
 Shevet Halevi 5/15