This article is a first draft summary of the Shulchan Aruch Harav and is pending further review of additional sources.
Seeing the hair of a woman:
The hair of a woman is an Erva, and hence one may not learn Torah or Daven in view of the hair of a married woman [or widow or divorcee]. [This applies to even if only a single strand of hair of the head is revealed.]
Gentile women: The hair of a gentile women is not an Erva.
A Besula: One may learn and daven within sight of the hair of a Besula [a virgin who was never married]. [The same applies for a woman who was never married even if she is a Beula.]
Sheital: A Sheital/wig is a valid head covering and one may learn Torah and Daven in view of a sheital.
According to the Tzemach Tzedek and Chazon Ish even a single hair of a married women in Erva.
Sideburns: Some Poskim rule that the hair of the sideburns is not considered an ervah in those areas that it is common to reveal. Other Poskim however rule that the hair of sideburns is considered an ervah and hence one may not Daven or learn in their view, with exception to one’s wife.
 Admur 75:4; Ketzos Hashulchan 9:3
Other opinions: Some Poskim rule that today since many married women have uncovered hair, one need not refrain from learning or Davening within its view. Other Poskim however argue that remains forbidden. [See Piskeiy Teshuvos 75:5]
 Michaber E.H. 21:2 and Rambam Issurei Biya 21:17 “It is forbidden for a married woman, or single girl, to walk in public with uncovered hair.”; The term single written above refers to a widow or divorcee, and they must cover their hair even when single: Beis Shmuel 21:5; Chelkas Mechokeik 21/2; Bach 21; Beir Heiytiv 21:5; Degul Merivava 21; Based on Yerushalmi Kesubos Halacha 1 who states that once a woman was married, she does not uncover her hair even if she became divorced or widowed prior to having marital relations
 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”; See Mahariy Levi 9 who allows revealing some of the hair until the day after the Chuppah.; 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
Other opinions: 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. [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 Rebbe, as well as the vast majority of today’s Poskim, negate this opinion, and state it has no place in Shulchan Aruch.
 Panim Meiros 1:25; Pischeiy Teshuvah 21/2
Other opinions: Some Poskim rule a Beula who was never married must cover her hair, even if this is due to rape. [Chelkas Mechokeik 21/2; Implication of Beis Shmuel ibid; Beir Heiytiv ibid; Degul Merivava 21]
 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”
 Tzemach Tzedek Even Haezer 139/2 and Mishnayos Brachos 3/5-3 [brought in Ketzos Hashulchan 9:4; 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.”