The definition of a Kosher Mechitza:[1]

A Kosher Mechitza is defined as a divider which prevents intermingling [i.e. talking and touching] between the two genders.[2] In addition, according to most Poskim[3], and so is the Rebbe’s opinion[4], it must also prevent the women from being in view of the men. Accordingly, a divider which prevents intermingling but does not prevent the men from seeing the women is invalid.

Height in a Shul: According to all opinions, the Mechitza must be at least tall enough to reach the average shoulder height of a person, which is approximately 18 Tefachim.[5] If it is lower than this height, then it is completely invalid. Now, regarding if it must be even higher than average shoulder height, is disputed amongst the Poskim. Some Poskim[6] rule that it suffices for it to be of shoulder height and remains valid even if the men can see the uncovered hair of married women, although it is an act of piety for it to completely block also the faces of the women.[7] However, according to most Poskim[8], and so is the Rebbe’s opinion[9], it must be tall enough to completely block vision of the women, and hence it must be tall enough to block the men from seeing the heads and faces of the women. At minimum, it must be at least 6 feet tall.[10] This applies even if all the married women cover their hair, and certainly if they don’t. Some Poskim[11] rule it should be high enough to prevent the vision of men even when they are standing on the podium, such as on the Bima or steps of the Aron Kodesh. [Nonetheless, effort should be placed to have a Mechitza of the height of at least the lenient opinion, if the congregation refuses a Mechitza of the stringent opinion, and with time perhaps one can influence them to make it higher.[12] However, one should not accept the position of being a Rav in such a congregation[13], and perhaps should not Daven in such a congregation, as such a Mechitza is invalid according to the Rebbe and many Poskim.]

Height by an event: Some Poskim[14] rule that only in a Shul is it required for the Mechitza to be the above height to completely block the sight of the other gender. However, by an event, such as a Sheva Brachos meal or Shiur, a simple divider of even ten Tefachim is valid.

Glass: A see through glass Mechitza is invalid.[15] Furthermore, according to most Poskim[16], and so is the Rebbe’s opinion[17], it is invalid even if the majority of the height of the Mechitza is not made of glass and there is simply a see-through glass partition placed on top which allows the head and face of the women to be seen. However, if the glass partition is tainted in a way that only the women can see the men but not vice versa, then it is valid.[18] Even this however, was discouraged by the Rebbe unless the women section is by an upper balcony.[19] Certainly, an opaque or stained or smoked glass Mechitza should not be used for the length of the Mechitza, if it does not completely block the site of people behind it, even if it blurs their body.[20]

Perforations:[21] It is permitted for the Mechitza to contain small perforations which allow the women to see through it to the men’s section. The holes must be small enough to block the men from being able to see the figure of a woman through it. If the holes are large enough for men to be able to see into the womens section, then it is invalid.

Balcony:[22] A balcony Mechitza [i.e. women are on the balcony] is valid. However, even by a balcony Mechitza, it is necessary to have a non-see-through partition to prevent the men from seeing the women when they look up.

Trees, plants, Filigree, lattice work: It is permitted for the Mechitza to be made of any item or material so long as it blocks the men from being able to see the figure of a woman through it. If the holes are large enough for men to be able to see into the womens section, then it is invalid.

Length:[23] The Mechitza must stretch from one end of the women’s section to the other, so the men cannot see the women from any part of the Shul.

Separate entrances:[24] Separate entrances to the men’s and women’s section must be made so the men and women can enter from their own entrance, in order to prevent intermingling in the entrance area and prevent the men from seeing the women there. [However, if this is not possible, then one may be lenient.]

[1] See Divrei Yoel O.C. 10 in great length in negation of the opinion of the Igros Moshe ibid; Likkutei Minchas Yitzchak 14; Tzitz Eliezer 7:8; Mishneh Halachos 7:12; Piskeiy Teshuvos ibid; Shulchan Menachem 1:106

[2] Igros Moshe O.C. 1:39

[3] See Divrei Yoel O.C. 10 in great length in negation of the opinion of the Igros Moshe ibid

Other opinions: Some Poskim negate this as being the reason behind the need for a Mechitza between men and women. [Igros Moshe O.C. 1:39]

[4] Rebbe in Igros Kodesh 7:309; 14:228; 19:73; 23:156; Otzer Hamelech 3:146

[5] Igros Moshe O.C. 1:39, 41-42 in negation of opinion that a Mechitza of ten Tefachim high and four Tefachim wide suffices [as is the law of a Mechitza by other Halachic matters]. This negated approach was seemingly the approach of Rav Y.B. Soloveichik, as brought in Mipinyanei Harav p. 46

[6] Igros Moshe O.C. 1:39, 41, 42; 2:40; 3:23-24

[7] The reason: As in their opinion, the main reason for the division is to prevent intermingling and not to block the view of the other gender. [Igros Moshe O.C. 1:39]

[8] See Divrei Yoel O.C. 10 in great length in negation of the opinion of the Igros Moshe ibid; Kaneh Bosem 2:9; Piskeiy Teshuvos ibid; All Poskim who rule the purpose of the Mechitza is to block the sight of the women: Maharam Shick O.C. 77; Zichron Yehuda O.C. 72; Divrei Yoel O.C. 1:10 in great length; Tzitz Eliezer 7:8 in name of 70 Rabbanim; Rebbe in Igros Kodesh 7:309; 14:228; 19:73; 23:156; Otzer Hamelech 3:146; Shevet Halevi 1:29; Beir Moshe 4:147; Mishneh Halachos 7:12

[9] Igros Kodesh 7:309; 14:228; 23:156; Otzer Hamelech 3:146

[10] Igros Kodesh 7:309; 23:156

[11] Kaneh Bosem 2:9

[12] See Igros Kodesh 14:228 for a similar response of the Rebbe, although to note that there it refers to a Mechitza that does reach head height of those women present, but not of all women, as explained in Pardes Chabad ibid

[13] See Igros Kodesh 23:156

[14] See Kiddushin 81a and Rashi ibid [brought in source #3]; Seridei Eish 1:77; Shevet Halevi 8:281; See Hiskashrus 588:11 that according to the Sefer Chassidim that the intermingling itself is forbidden, perhaps having separate seating suffices and having actual dividers is not necessary. However, according to the Sefer Minhagim Tirana, it is clear that a blocking divider is necessary

[15] Igros Kodesh 7:309; Shevet Haleivi 1:29; Divrei Yisrael, brought in Taharas Yom Tov 3:28; Tiferes Naftali 12; See Igros Moshe O.C. 1:43 that this is due to the fact that immodestly dressed women can be seen through it, and not due to an intrinsic prohibition. However, according to the opinions that require a Mechitza in order to block the view of women, then it is most certainly invalid in its own right; See Admur 75:3; All Poskim who rule the purpose of the Mechitza is to block the sight of the women: Maharam Shick O.C. 77; Zichron Yehuda O.C. 72; Divrei Yoel O.C. 1:10 in great length; Tzitz Eliezer 7:8 in name of 70 Rabbanim; Rebbe in Igros Kodesh 7:309; 14:228; 19:73; 23:156; Otzer Hamelech 3:146; Shevet Halevi 1:29; Beir Moshe 4:147; Mishneh Halachos 7:12

[16] See Divrei Yoel O.C. 10 in great length in negation of the opinion of the Igros Moshe ibid; Piskeiy Teshuvos ibid; All Poskim ibid who rule the purpose of the Mechitza is to block the sight of the women

Other opinions: Some Poskim rule that a glass Mechitza is valid if the women are dressed modestly. [Igros Moshe O.C. 1:39]

[17] Igros Kodesh 7:309; 19:74

[18] Igros Moshe O.C. 1:43

[19] Igros Kodesh 23:156; See Shulchan Menachem 1:106 footnote 13 that prior to the high holidays of 1973 they were unable to get stained glass for the women’s balcony, and hence had a regular glass divider and the Rebbe stated he would not go down to pray in the shul unless this was fixed.

[20] See Shevet Haleivi 1:29; Divrei Yisrael, brought in Taharas Yom Tov 3:28; Tiferes Naftali 12

[21] Maharam Shick O.C. 77; Divrei Yoel O.C. 1:10; Igros Kodesh 7:309; Piskeiy Teshuvos ibid

[22] See Igros Moshe O.C. 1:42; Igros Kodesh 7:309; Arugas Habosem O.C. 26; Seridei Eish O.C. 14

[23] See Sukkah ibid in Source #2

[24] Shaareiy Teshuvah 153:27; Mamar Mordechai 3-4; Chad Vachalak 2:59