Shulchan Aruch Chapter 10. Laws concerning the Corners of the Talis טלית
1. A טלית (or garment) without 4 corners is exempt from Tzitzit. If it has more than 4 corners, one is obligated to put Tzitzit on 4 of the most widespread corners.
2. If the garment had 4 corners and one corner was cut on the diagonal making two, it now has 5 corners and requires Tzitzit.
3. If the corners were folded over and tied or sowed in such a way that it appears they were cut off and no corners exist, none-the-less this does not make the garment exempt.
4. A טלית of cloth with corners made of skin requires Tzitzit. If the garment was made out of skin and the corners out of cloth, it is exempt.
5. Consider a three cornered garment with 3 Tzitzit (fringes). Then a fourth corner was formed and Tzitzit were affixed to the new corner. This is invalid due to the concept of תעשׂה ולא מן העשׂוי.
6. One cannot fold a טלית in two and put Tzitzit on its corners while it is doubled. [But it is necessary to put them on four completely seperate corners.] This is unless you sewed the entire garment even on one side. [Some say that the garment needs Tzitzit even if without sewing. It is a good idea to make Tzitzit for the garment but don’t make a blessing on it.] If one put Tzitzit on a garment that already had Tzitzit, if he originally had in mind to invalidate the first set, he cuts off the first set and it is Kosher. If he originally had in mind to add, even if he cuts off one of them, it is invalid. [Some say this is Kosher in all cases and such is the majority opinion. All agree that it is invalid before one set is cut off.]
7. There are garments with the sides open below, with 4 corners on the bottom. But on top they are closed. If the majority of the garment is closed, it is exempt (from Tzitzit). If the majority of the garment is open, it requires Tzitzit. If exactly half was open, we lean to the stringent side and it requires Tzitzit. One shouldn’t go out with this on the Sabbath (bec. of carrying).
8. Consider a cape, that has 4 corners when open. But if one attaches an אשׁטרינג׳ה to make it closed in order for the cape to be exempt from Tzitzit, this does not help unless it covers half of the length on the lower side. Similarly, it should be attached below the belt in order that most of the closed part should be visible to the naked eye. For if not, it is invalid (needs Tzitzit) because of מערת העין.
9. The corners must to be square and not circular.
10. A headdress is exempt form Tzitzit. Even those from the western lands where two parts are wrapped around the shoulders and body are exempt. Even if it is used to cover the head and most of the body, it is exempt since its main purpose is to cover the head. The Torah says, “your covering” and not one that covers your head.
11. A scarf that is worn around the neck in certain provinces, which is called שׁי׳ד (Shid) in arabic, and similarly a בוק׳א (Bowka) that the middle eastern provinces put on their shoulders, are exempt.
12. Egyption garments called גוחא׳שׂ (Goochash) and similarly מינטיג׳י (Mintigiy), דיאלמני׳ש, קאפטאני׳ש and פידיני׳ש from Turkey, even though these have four corners, they are exempt. [The same applies for the garments worn in the provinces of the Ashkenazim and Sefardim. Since there are no corners made where two are in front and two are behind facing one other, they are exempt.]
 “You must do and not from something already made.” The garment must first be completed before the Tzitzit are added. By adding a new corner, it is like you are making a new garment. Therefore, you must add all 4 Tzitzit after and not before making the new corner. By “invalid” he means that it is as though you had no Tzitzit and would suffer a Biblical prohibition by wearing such a garment. In such a case, you must remove all fringes made before the modification and put them back on.
M.B. writes that even if a garment had 4 corners with Tzitzit and one made 6 corners, it is invalid.
 M.B. explains that the Tzitzit must be on the corners and not in the middle of the garment.
 M.B: Some say that if one wears a folded garment that makes 4 corners, it needs Tzitzit on all four sides while it is doubled. This creates a big problem, for some say Tzitzit must be added on the 4 artificial corners while the S.A. claims that Tzitzit must be added on its four real corners (when it is unfolded). However, one can’t add 6 Tzitzit because of the prohibition of לא הוסיף. Therefore, one should not wear such a garment.
 M.B: “וכן פסקו האחרונים”
 M.B.: It is an article of clothing which attaches to the sides of the cape.
 Although the garment doesn’t need Tzitzit, it would appear to the onlooker that it needs Tzitzit. The top part of the garment would be under his clothes while an onlooker will only see the lower part that is mostly open.
 M.B. says that this is one of the reasons why our garments don’t have Tzitzit.
For example, although a dress shirt has 4 corners (two on the collar and two on the bottom) the corners always are in front of the wearer. You must be able to have 2 corners in front and two behind. However, the M.A. states that such an explanation is highly questionable and vague. Therefore, one who fears heaven should round off one of the corners on such garments (M.B.). Everyone agrees that if a suit jacket or coat, has a slit split more then half way down the back, it requires Tzitzit on its four corners (M.B.).