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The Corners: 
A. Four corners:
A Tallis that contains less than four corners is exempt from Tzitzis.
More than four cornered garment: A Tallis that contains more than four corners, such as a garment that contains five or more corners, is obligated in Tzitzis. Nevertheless, it is proper to beware to not make any garment more than four corners. A Tallis that contained four corners and had one of its corners cut diagonally, hence forming this individual corner into two corners, is now considered a five cornered garment and retains its Tzitzis obligation. Likewise, if one cut two corners diagonally, it is considered a six cornered garment [and retains its Tzitzis obligation]. [It goes without saying that if one folded the corners of his Tallis in a way that it now appears like two corners and then sewed it so it retain this form, the Tallis nevertheless remains obligated in Tzitzis.]
How to tie the Tzitzis on a more than 4 cornered garment: One is not required to tie Tzitzis to every corner of the garment, but rather to only four of its corners. Furthermore, it is even Biblically forbidden to tie Tzitzis onto more than four of its corners due to the prohibition of Baal Tosef. One is to tie the Tzitzis onto the four most distanced corners.
If the side of one’s Tallis tore and now contains more than four corners, is one to avoid wearing it?
If the Tallis did not tear in majority of its length or width, then one may wear it as usual. If however the Tallis tore in majority of its length or width then some Poskim rule one is to immediately fix the Tallis and resew the tear. Accordingly, one may not wear this Tallis with a blessing until it is sewn. Some Poskim rule one is required to undo all the Tzitzis and retie them after the tear is sewn, and the extra corners invalidated. Other Poskim however rule one is not required to undo the Tzitzis and the mere sewing suffices, and so is the ruling of Admur. This law applies likewise if the Tallis Katan tore in the neck hole and the tear extends for majority of the length.
Ruling of Admur in Siddur: According to Admur in the Siddur one is never required to resew the tear of a Tallis even if the tear forms more than four corners, unless the piece tore to the point that it is less than 6 cm attached, or is near the corner, as explained in Halacha 13B. See there!
If the square cloth added to the top of a Tallis Gadol became unsewn by some of its sides, are its corners obligated in Tzitzis and is one to avoid wearing the Tallis in such a state?
No. The Tallis remains valid even initially, [although is to be fixed due to the beauty of the Mitzvah].
B. Positioning of corners:
A four cornered garment is only obligated in Tzitzis if the corners are found at the natural ends of the garment. If however the corner is not the natural end of the garment but is attached on to the garment as an additional piece, then it is not obligated in Tzitzis. Thus, if one attached two square cloths to the middle of a garment, hence creating four corners, the garment remains exempt from Tzitzis.
C. A folded garment-Where to tie the Tzitzis; its real corners or its folded corners?
A Tallis that is very long, like the length of two Talleisim, and is folded in half and worn in this method, contains a question as to the identity of its corners that require Tzitzis. Is the Tzitzis to be tied only to the original corners, which are now all adjacent and parallel to each other and thus form only two corners, or is it to be tied to the newly created corners which now exist by the area of the fold? The following is the law:
Sewed the folded garment: If the folded Tallis has been sewed to the point that it can no longer be unfolded, two of the fringes are to be tied to the two new corners created in the folded area, and two Tzitzis to the two corners of the opposite side. This applies even if only part of the garment was folded. [However some Poskim argue that the above ruling only applies if the entire garment from end to end was folded and sewed on at least one side. If however only part of the garment was folded, such as its bottom edge was slightly folded and sewed, then the old corners retain their status. For this reason it is the custom of all Jewry to avoid making a fold at the end of the Tallis Gadol or Katan, in order to avoid the dispute of its proper corners. See Halacha 11E!]
Did not sew the folded garment: If the folded areas of the Tallis have not been sewed to the point that it cannot be unfolded, then if the Tallis is worn in this folded state, its identity of corners and consequent Tzitzis placement, is disputed amongst Poskim. Practically, being there is no final Halachic arbitration in this matter, therefore every person must beware not to wear a folded Tallis unless it is sewed after being folded [and then has its Tzitzis replaced on the new corners].
May one temporarily fold a Tallis he is wearing to prevent it from getting dirty and the like?
Yes. The above dispute only applies if one desires to keep the Tallis folded for an extended period of time. Nevertheless, even for temporary basis, one must beware not to fold the Tallis if doing so will cause it to lose its Shiur Tallis, as explained in Chapter 1 Halacha 11 A and D.
Is a very large Tallis that cannot be worn without being folded obligated in Tzitzis?
D. Round corners?
The corners of the garment that [are required to] have Tzitzis placed on them must be square and not round. Thus, if a four cornered garment has even one of its corners rounded it is not considered a four cornered garment and is hence exempt from Tzitzis. [Accordingly, in order to fulfill the Mitzvah and recite a blessing on a pair of Tzitzis, the corners must be square, and if the corner is round the Mitzvah is not fulfilled and a blessing may not be said. See Q&A regarding the law if the corner of a Kosher Tallis became rounded.]
Folding a corner until it is rounded and sewing it: If one folded the corners of his Tallis in a way that it now appears round, without a corner, and then sewed it so it retain this form, the Tallis nevertheless remains obligated in Tzitzis. The Tzitzis are to be attached to the original corner area that is currently folded and sewed.
How rounded must a corner be to be exempt from Tzitzis?
Some Poskim rule one is not required to round the entire corner area, but rather any roundness suffices [so long as its roundness is recognizable to others], and so is the custom. Other Poskim leave this matter in question. Others rule one must round the entire corner area, which is the entire 6×6 cm corner area. One is to thus begin cutting the circle from 6 centimeters from the bottom of the width and length of the corner. [see Exhibit A] This will form a circumference of 9 centimeters, which can be measured by placing a 9 centimeter thread around the circumference. A perfect circle can be accomplished by cutting out a circle that has a dimension of 12×12 centimeters [radius of 6 centimeters; See Picture] and then positioning it on the corner in a way that the edges of the circle sit exactly at the ends of the corner. One then cuts all the excess cloth that protrudes past the circle.
If the corner of one’s Tallis became rounded due to damage, is it still Kosher?
If one tied Tzitzis to a valid four cornered Tallis and a corner of the Tallis became rounded, either due to tearing or wearing out, the Tallis remains valid. Nevertheless, some write it is proper to fix the corner in order not to stumble the onlooker to think that such a Tallis is obligated in Tzitzis even initially.
The Tallis no longer has four Kosher sets of Tzitzis: If the above damaged corner that became round loses one of its Tzitzis, either due to it tearing off together with the corner or do to an invalidation which requires the Tzitzis to be changed, then the Tallis is invalid and prior to tying a new set of Tzitzis to the corner one must fix the rounded corner and untie all the other Tzitzis from the Tallis. Once the corner is fixed one is to retie all the Tzitzis to all four corners.
Is a four cornered garment that contains stuffing, judged as containing round or square corners?
- Example: A four cornered blanket that contains stuffing which causes the corners to protrude and form a rounded area.
The corner is considered round.
E. Sewing within the corner:
Some Poskim rule it is forbidden to sew within the legal corner dimensions, which is within 4-6 cm of the corner’s edge, any white thread which is made of a material that is valid to be used as Tzitzis for that garment. Thus, according to this opinion it is forbidden to sew the rim of the corner holes, for the purpose of preventing tearing, using a white thread which is a valid material for the Tzitzis of that garment. Accordingly, one may not sew the corner holes of a wool Tallis using white wool threads [and one may not sew the corner holes of a cotton Tallis using a white wool or white cotton thread]. Practically, although there are Poskim that argue on this opinion, nevertheless one is to be stringent if possible. According to all opinions, one may sew using a colored thread of any material even within the corner.
Sewing the rims of the holes: It is initially proper [and so is the custom] to sew a rim around the holes to which the Tzitzis are tied, in order to prevent the hole from tearing and having the strings fall below the 4 cm mark. Based on the above, one is not to use for this sewing a white thread which is of valid material for the Tzitzis of this garment. [Thus, on a wool garment one is not to use white wool threads to sew the corner. On a cotton garment one is neither to use wool nor cotton white threads, and is rather to use threads of other materials.]
Sewing the rims of the corner: It is initially proper [and so is the custom] to sew a rim onto the edge of the corners, in order to prevent the corner from tearing or wearing out, hence causing the hole to be less than 4 cm from the edge. [One may use any material thread and any color thread to sew this rim.]
May one enforce metal rings by the corner hole in order to protect it from tearing?
Yes. Likewise, one may use metal threads to sew the corner hole and edges.
Threads/loops/bells at the edge of the Tallis:
It is customary to place loops and bells on the edge of a Tallis Gadol [and in some Chassidic communities, also by the Tallis Katan] as a sign of beauty. Due to reasons explained in Halacha 12D one must beware that there are no loops or bells within 6 cm of any corner of the garment. It is for this reason that Tallis manufacturers only make the bells in the middle of the Tallis and distance themselves from adding any material in approximation to the corners. If there are bells or loops in this area of the corner, one is to cut it off.
F. Sewing an extra piece of corner cloth under each of the four corners?
It is customary to sew a piece of cloth by [each of] the corners of the Tallis. This corner may be made of any material, even leather. [The Chabad custom is for this piece of cloth to be made of silk. Based on the previous Halacha, when sewing this extra corner to the garment one is to beware not to sew within the 4-6 cm corner area using a white thread that is valid for the Tzitzis of that material Tallis. However, the edges of this corner cloth may be sewed onto the Tallis using any material thread.]
The laws relating to a torn corner-See Halacha 12!
 10/1; Michaber 10/1; Menachos 43b
 The reason: The verse [Devarim 22:12] states “Make for yourself Tzitzis on the four corners of your garment” The term four is coming to exclude three. [ibid]
Tallis of three corners was made into four corners: If a Tallis that contained three corners was cut into four corners, it is obligated in Tzitzis. Any Tzitzis that were tied prior to its fourth corner being created, is invalid and must be removed and then retied. This is due to the rule of “Taaseh Velo Min Hassuy”. [10/10; Michaber 10/5; Menachos 40b]
 10/1; Michaber 10/1; Menachos 43b
The reason: As the above verse superfluously states at the end “Asher Techaseh Bah/That you wear”. The Sages learn from these extra words that it is coming to teach that a five, or more, cornered garment is obligated in Tzitzis. Now, although it is possible to argue that this extra wording is coming to include a three cornered garment, while it is a five cornered garment that is meant to be excluded by the previous words of “four corners”, nevertheless logically there is greater reason to include a 5 cornered garment, which contains 4 corners, and exclude three cornered garments which do not contain four corners. [Admur ibid; Menachos ibid]
Other opinions: Some Poskim rule a five, or more, cornered garment is exempt from Tzitzis. [Zevachim 18b; Sefri Bamidbar Piska 115; Yireim 401; Hagahos Maimanis 3/2; Rokeiach 361; Or Hachaim Shlach; Bach 10; brought in M”A 19/3 and Admur 10/5] The Bach ibid explains that it is for this reason that the widespread custom of Jewry is not to place Tzitzis on various garments, as these garments usually contain five, and at times even six corners, and we hold like this opinion that exempts such a garment from Tzitzis. Practically, we do not rule like this opinion as stated above.
 10/5; M”A 10/3
The reason: Although according to the final ruling, a five or more cornered garment requires Tzitzis [on four of its corners], nevertheless since there are opinions [see above] which say that it is not required, therefore one should not wear a Tallis with five corners. [ibid] Vetzaruch Iyun as to why one should not make any of his garments contain five corners? What is wrong with wearing a valid four cornered Tallis Katan, and another garment of five corners that contains Tzitzis? Perhaps the reason for this is because one cannot recite a blessing upon wearing such a Tallis, and one should not enter himself into a situation of Safek Brachos. [See Chidushei Harim 1]
May a blessing be recited on this Tallis if one chooses to wear it? No. One is not to wear such a Tallis with a blessing due to Safek Brachos Lihakel. [Kaf Hachaim 10/1 in name of Pnei Yitzchak; Chidushei Harim ibid; Toras Chaim Sofer 10/1] Vetzaruch Iyun from Admur in Siddur which does not even require one to resew a tear in the Tallis so long as it remains 6 cm attached. See Halacha 12, and Q&A here.
 10/5; Michaber 10/2; Menachos 37b
 Admur ibid; M”A 10/4
How much of the corner must be cut in diagonal for the two corners to have a Halachic status of two corners? See Biur Halacha 10/2 “Yeish Lah”; Piskeiy Teshuvos 10/3
 See 10/6
 10/2; Michaber 10/1
 The reason: Being that the entire inclusion of a more than four corner garment is due to the fact that it also contains four corners, therefore only four of its corners are obligated in Tzitzis. [Admur ibid]
 Admur ibid; M”A 10/2
How to validate the Tallis if one transgressed and tied Tzitzis to all five corners: See Chapter 4 Halacha 7C!
 10/3; Michaber 10/1; Taz 10/2
The reason: As the verse states “On the four corners” and the corners most distanced from each other are the most recognizable corners. [ibid] This implies that according to Admur this requirement is Biblical, as rules the Elya Raba in name of the Yireim and Lechem Chamudos. Other Poskim however rule it is only Rabbinically required. [see M”B 10/5]
The law if one did not tie it to the most distanced corners: This matter is dependent on whether one learns the requirement of distanced corners is Biblical or Rabbinical, as explained above. The M”B ibid concludes that in a time of need one may be lenient to wear the Tallis.
 As a corner is only defined as a Halachic corner if it is split for majority of the length of the Tallis, as explained in Halacha 6.
 Chesed Lealafim 10/4; Ben Ish Chaiy Noach 8; Kaf Hachaim 10/7
 The reason: As this Tallis now contains more than 4 corners which is initially not to be done, as explained above.
 So rules regarding any 5 cornered garment: Kaf Hachaim 10/1 in name of Pnei Yitzchak; Chidushei Harim 1
 Chesed Lealafim 10/4; Ben Ish Chaiy Noach 8; Kaf Hachaim 10/7
 Toras Chaim Sofer 10/1; Makor Chaim of Chavos Yair; Piskeiy Teshuvos 10/2 footnote 15
 Admur 15/10 and in Siddur rules that so long as 6 cm remain attached it may be resewed with the Tzitzis.
 M”B 10/3 in name of Peri Megadim
 Siddur Admur [letter 20 in Raskin] in stringent opinion of Tosafus that when it retains 6 cm “It is then valid even without sewing”; Ketzos Hashulchan 6/10
 As a) It does not contain Shiur Tallis and b) It is not worn for a purpose of clothing and c) If it is in the inner part of the Tallis then it is exempt because we always follow the outer garment. [See Halacha 1C; Admur 10/9]
 10/23; M”A 10/13
 The reason: Certainly such corners are exempt from Tzitzis as these are not the corners referred to in the Torah. [ibid]
 10/13; Michaber and Rama 10/6; Menachos 41a
 Admur ibid; Michaber ibid; Menachos ibid
May one place two Talleisim one on top of the other and then tie Tzitzis to their corners? See Piskeiy Teshuvos 10/7 that one may do so,.
 This applies even if the sewing was only done to one side of the Tallis, if it is now impossible to unfold it. [Admur ibid based on Michaber ibid]
How many sides of the Tallis must be sewed? The Michaber states “Unless it was completely sewed even from one side”. This wording is contradictory, as if it is only sewed on one side then it is not completely sewed. The Biur Halacha ibid explains that perhaps the word “one side” in the Michaber is going on the top or bottom, although every direction of the Tallis must be sewed. However Admur ibid [as well as the Levush and Elya Raba] omits the word “completely” and simply writes it is to be sewed on one side. This implies that he learns the main ruling is that so long as one side is sewed it suffices. So rules also Aruch Hashulchan 10/7 who negates the explanation of the Biur Halacha. Other Poskim however rule that it must be sewed on three sides. [Artzos Hachaim; Biur Halacha 10 “Ela Im Kein”]
 This applies according to all opinions, as the sewing enforces its new dimensions, and the true corners of the Tallis in this state include the two corners of the folded area. [ibid]
 Admur 10/6; M”A 10/4; Levush; brought in M”B 10/8
 See M”B 10/8 in name of Artzos Hachaim; Machatzis Hashekel; Aruch; Mordechai; Perisha
 Poskim in previous footnote rule one is to follow the old, unfolded corner dimensions in this case. However the M”A and Levush rule one is to follow the new dimensions, and so is the ruling of Admur ibid. [See M”B ibid; Piskeiy Teshuvos 10/6]
 Piskeiy Teshuvos 10/4; Kaf Hachaim 10/8
 Admur ibid
 Some Poskim [Tana Kama Menachos 41a; Rosh; Rama ibid; first opinion in Admur ibid] rule we follow the folded state of the Tallis when it is worn, and hence two of the Tzitzis are to be tied to the two corners of the folded area, one on each side, as this is the defined corner of a folded Tallis during the time of wearing. The other two Tzitzis are to be tied to the other end of the Tallis, which now contains two sets of corners. The Tzitzis is to be tied to one of the corners of each set. [Admur ibid]
Other Poskim [Rebbe Shimon ibid; Rambam 1/17; Michaber ibid; second opinion Admur ibid] however rule we follow the unfolded state of the Tallis, as the fold was not sewed and is meant to be unfolded, and hence constantly maintains those dimensions even when folded. Therefore, two of the Tzitzis may not be tied to the two corners of the folded area, being that when the Tallis is unfolded these Tzitzis will be positioned in middle of the Tallis. Rather all four Tzitzis are to be tied to the natural corners when the Tallis unfolded. [Admur ibid]
 Admur ibid; M”A 10/7
Other opinions: Some Poskim rule the main opinion is like the Michaber ibid that we follow the original corners of the unfolded state. [Shaar Hatziyon 10/27 in name of Artzos Hachaim and Maharam Bennet; Mishkanos Yaakov] Some Poskim rule one is to tie 6 sets of Tzitzis to this Tallis, four to the unfolded corners, and two to the folded corners. [P”M 10 A”A 7]
 Piskeiy Teshuvos 10/6
 See Taz 10/7; P”M 10 M”Z 7; M”B 10/16
 Admur 10/13 writes “A Tallis that is the length of two”, and seemingly he is coming to teach that it is obligated in Tzitzis.
 10/19; Michaber 10/9
 The reason: As a round shape is not considered a corner. [ibid; Levush 10/9]
 10/22; 18/5; M”A 10/13; 18/3
 10/6; Michaber 10/3; Menachos 37b
If the corner was professionally sewed in a way it forms a corner: See Biur Halacha 10/3 “Lo” ; Rav Poalim 3/1; Aruch Hashulchan 10/8
 The reason: As so long as one has not completely cut the corner off the Tallis, he shows that he still desires it, and plans to eventually return the fold to its cornered state and hence the fold is still considered as part of the Tallis. Therefore, the Tallis legally retains its four corners and is obligated to have the Tzitzis tied to the original area of the corner. [Admur ibid]
 Admur ibid; M”B 10/8
The reason: As although it is now sewed to the garment, nevertheless since it will return to its original state, it is considered the fourth corner. [Admur ibid] Seemingly this is coming to exclude tying the Tzitzis to a lower area of the newly rounded corner. One thus ties the Tzitzis to the same dimensions of the original corner prior to the folding.
 See Biur Halacha 10/9 ”Velo”; Piskeiy Teshuvos 10/9
 Ashel Avraham Butchach 10
 Biur Halacha ibid
 Mor Uketzia 10; Ketzos Hashulchan Vol. 1 in supplements p. 85b; Maharil Diskin in Toras Haohel, brought in Ketzos Hashulchan ibid; Pischeiy Teshuvah 10; Or Letziyon 2/2 footnote 4
 Ketzos Hashulchan in next footnote; Maharil Diskin in Toras Haohel, brought in Ketzos Hashulchan ibid
Other opinions: Some rule one is to begin the circle from 7.2 centimeters from the bottom. [Shiur Chazon ish]
 Ketzos Hashulchan Vol. 1 in supplements p. 85b
 Divrei Menachem 10/1 on Tur; Kaf Hachaim 10/2; Piskeiy Teshuvos 10/1
 The reason: As we are only particular that the corners not be round at the time of the tying of the Tzitzis to the Tallis. Once however the Tzitzis has been tied to a validly four cornered Tallis, the rounding of a corner does not invalidate it. This is similar to the law brought in Halacha 11 regarding the Halachicly valid dimensions of the corner, which only apply prior to the tying of the Tzitzis, and if the Tzitzis tears and is now hanging from the wrong dimensions, it nevertheless remains Kosher. [Kaf Hachaim ibid; See Piskeiy Teshuvos ibid footnote 4]
Ruling of M”B: The M”B 10/1, 10/12 and 15/6 rules that if the corner became rounded, the Tallis is invalid and all the Tzitzis must be removed and retied after the corner is fixed. After closer analysis however it is evident that the M”B in truth is not arguing on the above Poskim, as the M”B ibid is referring to a case that the Tallis no longer contains four sets of Tzitzis by its corners, and hence since one now desires to tie a fourth Tzitzis to its corner, one is required to undo all the other Tzitzis due to Taaseh Velo Min Hassuiy. However, so long as the Tallis still retains its four Tzitzis it remains valid even with a rounded corner, even according to the M”B, just as is the law with a torn corner hole that has the Tzitzis remaining attached, as explained in 11/18 and Halacha 11B. [Piskeiy Teshuvos ibid footnote 4 unlike Beis Baruch 11/50]
 Piskeiy Teshuvos ibid footnote 5 based on 11/10 [Admur 11/18]; To note however that even in 11/18 it does not mention that one is to resew the corner area after tearing, and simply mentions that one is to sew it to prevent tearing. Likewise, the Kaf Hachaim ibid makes no mention of this need to fix the corner.
 M”B 10/1; 10/12; 15/6; Piskeiy Teshuvos ibid; see previous footnotes
 See Piskeiy Teshuvos ibid footnote 6 that this only applies if the Tzitzis of the torn corner tore off together with the corner and not if the Tzitzis of a valid corner is removed and replaced. Vetzaruch Iyun, as once one of the Tzitzis is removed the Tallis is invalid, and once it is invalid it should revert to its three corner status of invalidation and hence require all its Tzitzis to be removed and retied just as we rule above in A, that one who made Tzitzis on a three cornered garment must remove all the Tzitzis before making the 4th corner. See also Admur 15/8 which implies in his reason that in all cases, all the Tzitzis must be removed if the garment does not contain 4 corners. Vetzaruch Iyun.
 Ashel Avraham Butchach 18/2
 15/16; Michaber 15/6; see Halacha 13F
 Rashi Menachos 41a; Rosh 2/11
 Admur ibid; Michaber ibid; see Halacha 12A and 13F]
Other opinions: Some Poskim rule that according to Rashi ibid one is to avoid any thread that is valid for Tzitzis even below 4 cm from the corner. [Chazon Ish 3/19]
Ruling of Admur in Siddur: See Siddur Admur [letter 20 in Raskin] that writes that if a tear occurred outside the corner area one is to sew using a non-Tzitzis material thread. Vetzaruch Iyun Gadol! Rav Raskin in footnote 107 leaves this matter inconclusive!
 The reason: These Poskim rule it is forbidden to sew within the legal dimensions of the corner [4-6 cm] any thread which is a Kosher material of Tzitzis for that clothing. The reason for this is because one may come to use one of these threads to make the Tzitzis. This is forbidden being that the thread was not woven for the sake of Tzitzis. [Admur ibid; M”A 15/5] Other Poskim however rule the reason this is forbidden is because of Taaseh Velo Min Hasuiy. [Rashi ibid; Aruch Hashulchan 15/10]
Is the Tallis Pasul Bedieved according to this opinion: Some Poskim rule that if one sewed using valid Tzitzis material within the corner dimensions, then according to Rashi the Tallis is invalid even Bedieved. [See Biur Halacha 15 “Eicha”]
 As wool Tzitzis is valid for any material garment.
 Admur ibid; Michaber ibid
Other Poskim: Some Poskim rule that in today’s times that the custom is to always use wool Tzitzis strings on all material garments, it is permitted to sew within the corner dimension with a thread that is the same material as the garment. [Aruch Hashulchan 15/10 and 16 as rules Admur next regarding a colored thread] It is based on this opinion that Tallis manufacturers are lenient to use cotton threads to make the hole of even a cotton Tallis Katan, being that the Tzitzis is always made of wool. Practically however, one is to try to be stringent in this matter. [Piskeiy Teshuvos 15/6]
 Admur ibid; M”B 15/18 in name of Derech Hachaim; Aruch Hashulchan 15/16; Kaf Hachaim 15/36
The reason: There is no need to suspect that one may come to join a colored thread to the other Tzitzis strings, as in today’s times we are no longer accustomed to ever make colored Tzitzis. [Admur ibid; Poskim ibid]
Other opinions: Some Poskim argue that one may not use a colored thread if it is made of the same material as the garment. [Chazon Ish 3/19]
 Rama ibid
The reason: Although even if the hole were to tear, the Tzitzis remains Kosher, as explained in Halacha 12B, nevertheless initially one is to prevent this from happening in order so people do not think that the hole was like this before the Tzitzis were tied [in which case it would be invalid]. [ibid; 2nd reason in Beis Yosef] The M”B 11/51 records the 1st reason in Beis Yosef that “So people don’t come to think it is invalid.” Other say the reason is because it is not a Hiddur Mitzvah to have the Tzitzis left in a torn fashion. [Aruch Hashulchan 11/14]
 15/16; Michaber 15/6
 11/18; Rama 11/10; Smak Kol Bo
 Rama ibid
 The reason: Although even if the hole were to tear, the Tzitzis remains Kosher, as explained in Halacha 12B, nevertheless initially one is to prevent this from happening in order so people do not think that the hole was like this before the Tzitzis were tied [in which case it would be invalid]. [ibid]
 As the sewing is not taking place within the legal corner dimensions of 4-6 cm from the edge.
 Minchas Yitzchak 3/2
 See Halacha 12D
 15/14; M”A 15/14; Ketzos Hashulchan 6/12-13
 The reason: This is done in order to verify that the corner contains a single piece of garment that is at least 6×6 cm. [Back then], the majority of the garments, even the new garments, contained sewed [pieces of garment] within the corner area, which is three Gudlin from the edge of the corner. Now, according to some Poskim [explained in 15/12], there may not be any patch of clothing [which is less than 6×6 cm] sewed within 3 Gudlin from the corner, in the area that the Tzitzis are tied to [and if one sewed a patch to that area, according to this opinion, the patch is considered to be not attached to the Tallis, and hence if one ties Tzitzis to it, the Tzitzis is invalid]. Therefore, we place in the area that the Tzitzis are tied a cloth that is 6×6 cm and does not contain any patches, in order so the Tzitzis hang on an area that is fit to be called a corner [according to all]. [ibid]
The law if the corner tore and the Tzitzis is hanging on this corner piece: This extra piece of cloth is helpful even today in a case that the corner tore and needs stitching, in which case the backup corner serves as the Halachic corner of the garment. [See Piskeiy Teshuvos 15/5 footnote 35; next Halacha B in Q&A!]
Where to sew this extra piece: This corner may be sewed to either side of the Tallis, although the custom is to sew it onto the inner side.
 As even the leather corner of a garment is obligated in Tzitzis, as explained in 10/8
 Igros Kodesh 2/33 [printed in Shulchan Menachem 1/44]; This was the custom of Beis Rabbeinu