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Placing an extra cloth on the top area of the Tallis Gadol: 
It is customary to make a crown on the Tallis through sewing a piece of silk [or other material on its top area]. The Arizal however was not careful in this matter. [Based on the above custom, some wear Talleisim that have an added cloth with designs on the top. Others however, following the Arizal, do not attach any cloth to the Tallis. The Chabad custom is to sew a cloth on the inner side of the upper part of the Tallis, and not to sew anything on the outer side of the top part. So was also the widespread custom in many areas in Europe.
Placing an extra cloth on the corners of the Tallis: It is customary to place a silk cloth on the corner of each of the 4 corners of the Tallis. See Halacha 11F!
Should a Tallis Katan also contain a sign on its front part?
Placing a silver crown on the top section of the Tallis:
Some are accustomed to sew a gold or silver crown on the top part of the Tallis. In addition, some also sew a silver lining in the middle of the Tallis. Others however have discouraged this custom entirely. The Chabad custom is not to sew anything on the outer side of the Tallis.
 Admur 8/9; M”A 8/6 based on Rashal and Shlah Chulin; M”B 8/9; See Aruch Hashulchan 8/10 that writes this custom is a relatively new custom.
Other opinions: Some rule one should not place any additional cloth onto the top area of the Tallis, as in truth the main part of the Tallis is the area that covers the body and not the top. [Levush 8/10] See below in custom of Arizal, that he also opposed this custom.
 In Shaar Hakavanos Tzitzis 2 it states that this is an Ashkenazi custom [not the Sephardic tradition], and so is clearly apparent from the Kneses Hagedola 8
 Aruch Hashulchan 8/10
 The reason: This is done to serve as a sign towards which side of the Tallis is its front, in order so one always wear the Tallis with its top part on one’s head. The reason why one should always wear the Tallis the same side, is in order so the Tzitzis found by the two upper corners remain always in front of him and not [in back of him] towards the end of his body, as one only goes higher in holiness and may not diminish the holiness of an item [Maalin Bekodesh Velo Moridin]. [Admur ibid; based on Rashal and Shelah ibid]
Other reasons: Alternatively, the front Tzitzis should always remain in front similar the Northern beams of the Mishkan which were always to remain in the North. [M”A ibid in name of Shlah; brought in M”B ibid; Vetzaruch Iyun why Admur omitted this matter. See Adnei Paz 8/5 that negates this reason] This matter likewise applies to the top part of the Tallis itself, that the top material should always remain on top and not descend in Kedusha. [Olas Tamid 10/9] Alternatively, it is done in or to beautify the Tallis. [Levush 8/10] Alternatively, it is done to prevent the top part of the Tallis from wearing out. [Kneses Hagedola 8]
 Custom of Arizal in Peri Eitz Chaim Shaar Hatzitzis 1, brought in Admur ibid; M”A ibid regarding not being careful to wear the Tallis the same way each time, which is the reason behind making the silk attachment, as explained above. Nevertheless, it is unclear if this statement of Admur also refers to not placing a piece of silk on the top part of the Tallis. The Aruch Hashulchan 8/9 states, that based on the above one can assume the Arizal did not place a cloth there, as in such a case at times the Tallis would face with this attached cloth to the outside, which is a shameful matter. Afterwards however I found explicitly in Shaar Hakavanos Tzitzis 2 [and Matzos Shmurim; Nagid Mitzvah] that there is no need to be particular to have any cloth on the top of the Tallis as this custom has no root.
 Imrei Pinchas of Koritz; In Shaar Hakavanos ibid it states that this is not the Sefardi custom, and so is apparent from the Kneses Hagedola 8
 Ishkavta Direbbe p. 22 footnote 17; Rav Gurary in Chikreiy Minhagim p.17
The reason: The reason for our custom is in order to suspect for all the opinions on the matter. Hence on the one hand we do not sew anything on the outer top part of the Tallis, to suspect for those Poskim [brought above] that rule against doing so, being it appears as if one is showing that the head is the main part of the Mitzvah. On the other hand we attach a cloth to the inner side of the top part in order to have a sign on and not come to wear it in the opposite order, as rules Admur ibid. [Ishkavta Direbbe ibid] Alternatively, the reason this is done is in order to prevent sweat from ruining the actual material of the Tallis, and hence we sew the extra piece to the inner side, so it receives the sweat, and at the same time serve also as a sign to which part belongs on top. [Aruch Hashulchan ibid]
 Aruch Hashulchan 8/10
 See 15/14
 Piskeiy Teshuvos 8/13
 Some have a V-neck opening by the front side of the neck hole; Others have loops on the front side of the Tallis Katan; Others have different materials sewed to the front corners of the Tallis Katan.
 The Chabad custom: The Chabad Tallis Katan of today has no sign on it, either by the back or the front. It interesting to note that Admur ibid changed the wording of the M”A ibid as to the importance of this sign, stating that the Tzitzis which are in front should not be on bottom, hence implying that the main issue is not only that the front Tzitzis remain in front but that they should [mainly or also] not descend to the lower part of the body. Now, this is relevant only by a Tallis Gadol, as by a Tallis Katan both the front and back are equal in height. Vetzaruch Iyun!
 Minhag brought in Aruch Hashulchan 8/10 and Artzos Hachaim 8/4; Minhag Chassidim brought in Ishkavta Direbbe; See Igros Moshe 5/20
 The Artzos Hachaim ibid and Aruch Hashulchan ibid state that some make a silver lining also in the middle of the Tallis in order to show that also the middle is the main part of the Mitzvah; Darkei Chaim Veshalom 36
 Levush 1010; Artzos Hachaim 8; Aruch Hashulchan 8/10 “There is no room for gold or silver on the Tallis”; The Sephardic custom is not to have silver or gold lining on the Tallis. [Shaar Hakavanos Tzitzis 2; Kneses Hagedola 8; Adnei Paz 8/5]
 The reason: As the above custom shows that the main part of the Tallis is the top part by the head, when in truth the main part of the Tallis is the area that covers the body. [Levush ibid; Artzos Hachaim ibid; Aruch Hashulchan ibid]
 Ishkavta Direbbe p. 22 footnote 17; Chikreiy Minhagim p.17
The reason: The original custom of Chassidim, including Chabad Chassidim, was to sew a gold or silver lining on the top of the Tallis. When the Alter Rebbe was imprisoned in 1798, the Vaad Hatzalah decreed on all the Chassidim to remove and sell this silver lining in order to raise money to help redeem the Alter Rebbe from imprisonment. Seemingly, from that time and onwards the custom became to no longer sew silver onto the Talleisim. [Ishkavta Direbbe ibid] Some however assert that certainly the Alter Rebbe himself did not have a lining on the top of his Tallis, and that is the reason Chassidim did not do so. [Chikrei Minhagim ibid]