Sources on Tzeniut

Hagahot Maimaniyot Laws of Shema 3

And the Behag explained, this is in regard to his wife, but for another woman even less than a hand's breadth.

Talmidei R. Yonah Berachot 17a

And Rav Hai Gaon z"l wrote that the same law applies to any woman who is covered with a tefach in a covered place, that it is prohibited to read before her, because a tefach in a woman is ervah.

Pri Megadim, Mishbetzot Zahav 75

The entire leg until the limb we call a "knee" - in a place where they walk barefoot and revealed there is possibly no concern; and in a place where it is typical to cover them by a hand's breadth, and from the knee and up even less than a hand's breadth is also prohibited.

Mishnah Berurah 75

(2) To cover it - But her face and hands follow the practice in which is is typical to be revealed in that location. And similarly for the toes until the shok [which is until the place called knee in German language] - in a place where is is typical to walk barefoot, it is permissible to read before it: because he is accustomed to it, they do not lead to illicit thoughts. And where it is typical to cover them, the minimum amount is a hand's breadth, just as the rest of a woman's body. However, her arms and shok, even if they are accustomed to walk uncovered in the way of the unbounded, is prohibited.

Rokeach Laws of Prayer 324

And hair, whether of his wife or another woman, or her shok, or a hand's breadth of her flesh, or with her arms revealed, is prohibited.

Rashba Berachot 24a

And that which Rav Yitzchak said that a hand's breadth is nakedness for a woman, and we establish it in regard to his wife and for the recitation of Shema: The Ra'avad explained that it is possible that this refers specifically to a generally covered part of her body. And in that case Rav Chisda comes to say that the shok of a woman is considered a private and naked place...

Beit Yosef O.C. 75

And I was perplexed, what does Rav Chisda come to teach? Any place that is normally covered is ervah! And [so] why does it speak about shok? Until I found the Rashba... And our rabbi [the Tur] appears to follow this view. And this is what he wrote, a revealed tefach in a woman in a place that it is normally covered, which implies that in a place where it is not typical to cover, such as her face or hands, this does not apply. And afterward he wrote: "And similarly if her shok is revealed it is prohibited." In other words, even though it is not a private place for a man, it is ervah for a woman.

Turei Zahav O"C 75

And some say that specifically in regard to his wife etc. - The Tur wrote "and similarly if her shok is uncovered" - it would appear that this comes to teach us that even in regard to his own wife, and less than a hand's breadth it is prohibited, for it is a place of stray thoughts even more so than other limbs.

Mishnah Berurah Siman 75

(7) Less than a hand's breadth - And if the shok is revealed, some say that even for one's wife and less than a hand's breadth it is prohibited to read before it, for it is a place of illicit thoughts more than other limbs.

Chaye Adam 1:4:2

However, her arms and shok, even if they are accustomed to it in the manner of the unbounded, is prohibited.

Responsa Divrei Yatziv E"H 37

[One's sleeve length] depends on the practice of the community even with regard to her revealed arms, only in regard to a place in which it is normal to cover, and only a tefach. As opposed to the knee and above, we are more strict, that even less than a hand's breadth...