Placing the Tefillin by the hairline:
The entire Tefillin must rest in the area where hair grows. This means that [even] the bottom end of the Titura must rest on the area where the hair first begins to grow on the forehead. [One who is not careful in this matter and allows the Tefillin to breach below the forehead does not fulfill his obligation of wearing Tefillin. This applies even if only part of the Tefillin reaches below the hair. Unfortunately, many stumble on this prohibition and mistakenly think that the upper end of the Tefillin is to be placed in the area where the hair starts, while the main bulk of the Tefillin is to be placed on the forehead. Such a person transgresses the Biblical prohibition, as the entire Tefillin must be in an area with hair. Likewise, if they said a blessing, the blessing was said in vain. Accordingly, the many individuals who are not careful in this matter, are to be made aware, and each person is to remind his friend. If one ever feels that the Tefillin has fallen out of place, he is to immediately return it.]
No part of the Tefillin may rest below the hairline. One is to make others aware of this matter if he sees their Tefillin crouching below the hairline.
If one wore the Tefillin past the hairline, must he replace it with a blessing?
If he removed it with intent to not wear anymore, then it is to be replaced with a blessing of Al Mitzvas Tefillin. He is to repeat Kerias Shema while wearing the Tefillin.
The Rebbe’s Tefillin:
The Rebbe did not contain hair in the front portion of his head, and hence, when he wore his Tefillin, it appeared to many as if it were below the hairline, reaching into the forehead. Various unsubstantiated statements have surfaced as to the Rebbe’s remarks regarding this. Of these, the Rebbe is quoted to have remarked after being questioned, that in truth he does keep his Tefillin at the right area, and it is just that his hairline is not apparent due to lack of hair. In addition, the Rebbe is quoted to have stated that at times the Tefillin slides down and he constantly lifts them back up. Rabbi Leibal Groner informed that the Rebbe was once questioned as to whether one may be lenient to allow the Shel Rosh to go beneath the hairline, based on the fact that they see at times that his Shel Rosh falls below the hairline. The Rebbe responded to the questioner that he should follow the rulings in Shulchan Aruch, as stated above.
The Tefillin of the Belzer Rav:
It is said of the Belzer Rav, Rav Ahron of Belz, that he would his Tefillin would extend below the hairline. I heard the following story relating to this from an old Shamash at the Batei Rand Shul in the Nachalaot, Jerusalem neighborhood: It was on a certain occasion that the famed Gaon of Tzebin, Rav Dov Berish Weinfeld was invited to a daytime wedding in which the Belzer Rav attended. Upon seeing the Belzer Rav’s head Tefillin out of place, below the hairline, the known Gaon approached him and respectfully admonished him for wearing the Tefillin below the valid area. The Belzer Rav replied that indeed according to Shulchan Aruch it is not in the valid place, but that is what he received from his Chassidic masters. A certain Chassid of the Belzer Rav who had witnessed the conversation, could not hold back his fury at the Tzebiner Rav for admonishing his Rebbe, and when the Tzebiner Rav put down his hat on a chair, the Chassid quickly made the hat disappear out of vengeance for his Rebbe’s honor. Eventually, the Tzebiner Rav noticed his missing hat and asked for it to be returned. He added, that the Chassid who stole his hat and does not return it “Lo Yotzi Shenaso/You will not survive the year” When the Chassid heard this curse from the famed Gaon of Tzchebin, he immediately returned the hat, and later on had an audience with his Rebbe, the Belzer Rav, to discuss a Tikkun for his sin, and curse which he received. The Belzer Rav replied “You don’t understand what he said. He did not say that you will not survive the year, but rather that your Shenaso, which an also mean sleep, will never be fulfilled.” Indeed, this Chassid would fall asleep everywhere he went, and never became satiated from his sleep, as interpreted the Belzer Rav.
 Admur 27:14 [omitted in Siddur]; Michaber 27:9 “The area for placing the Tefillin Shel Rosh begins from the area that hair grows on one’s forehead”; Tur 27:9; Rambam Tefillin 4:1; Rosh Tefillin 19; Menachos 37a “Bemakom Sheoseh Karcha”
 Admur ibid; Taz 27:10; All Poskim in next footnote
 Admur ibid; Taz 27:10 as understood by some Poskim; Beis Yaakov 131; Elya Raba 27:10; Divrei Yosef 63; Yad Ahron on Hagahos Hatur; Soles Belula 27:3; Shalmei Tzibur p. 37; Birkeiy Yosef 27 Shiyurei Bracha 1; Kesher Guda 3:10; Siddur Beis Oved; Mamar Mordechai 27:8; P”M 27 M”Z 10; Shaareiy Teshuva 27:15; Kitzur SHU”A 10:3; M”B 27:33; Kaf Hachaim 27:38; Divrei Chaim 2:6; Likkutei Maharich; Os Chaim Veshalom 27:15; Maharshag 1:7
Other opinions: Some Poskim rule that only the Ketzitza [Bayis] must be placed above the hairline while the Titura may be placed below the hairline. [Taz 27:10 as understood by Beis Yaakov 131, although argues on the Taz; Gan Hamelech 153; Olas Tamid 27:8; Implication of Elya Raba27:10, brought in P”M and Kaf Hachaim ibid; Daas Kedoshim 27] Other Poskim rule that if majority of the Tefillin is above the hairline, it is valid. [Birchas Habayis 38:18; Devar Yehoshua 4:11 that so was custom of some Tzadikim; Ashel Avraham Butchach 27 in his initial suggestion, although does not conclude this way] See Os Chaim Veshalom and other Poskim ibid who completely negate this approach
Opinion of Taz: The Taz ibid records “The end of the Ketzitza is to be where the hair begins to grow.” Some Poskim understand the Taz to mean that the Titura may be below the hair, and only the actual Bayis must be by the hair. [Beis Yaakov 131; Elya Raba 27:10; Yad Ahron on Hagahos Hatur; Shalmei Tzibur p. 37] Other Poskim, however, learn the Taz to mean that the entire Tefillin must be above the hair. [Mamar Mordechai 27:8; P”M 27 M”Z 10; Shaareiy Teshuva 27:15; Kaf Hachaim 27:38]
 Taz 27:10; Shiltei Giborim on Mordechai Shabbos; Kneses Hagedola 27; Olas Tamid 27:8; Elya Raba 27:11; Soles Belula 27:4; Tefila Ledavid p. 20; Shalmei Tzibur p. 37; Chesed Lealafim 27:7; Kaf Hachaim [Falagi] 10:20; Shesilei Zeisim 27:23; M”B 27:33; Kitzur SHU”A 10:3; Ketzos Hashulchan 8:10
 Taz ibid; Chayeh Adam 14:10; Ben Ish Chaiy Chayeh Sarah 1; Kaf Hachaim 27:40; Poskim ibid
 Taz 27:10; Chesed Lealafim 27:7; M”B 27:33 “One who is not careful in this is included in Posheia Yisrael Begufan and is a Karkafta who has not put on Tefillin…It is as if the Tefillin is in his pocket.”
 M”B 27:33
 Yifei Laleiv 2:2; Shevet Hamussar 20 p. 51; Soles Belula 25; M”B 27:33
 See Ashel Avraham Butchach 27
 As in all stories, especially those heard second hand, not all details may be accurate to that which actually occurred, although the general story is reliable to have happened.
 See the “other opinions” brought in previous footnotes.